1. Facebook can look up mobile phone numbers

One of the features that work on some peoples' accounts but not others' is the ability to find profiles associated with a mobile phone number. Just type it into the search box. If it doesn't work, try adding the country code or log into another Facebook account.

2. You can find hidden profiles by guessing email addresses

You can usually find someone's profile by searching for the email address they used to sign up for Facebook.

Email addresses can sometimes be found online. Corporate email addresses usually run to a format and can be worked out.   Many people will use one of the top free email providers and, if paired with their name or regular online username, you can have a pretty good guess when trying to find a tricky profile.

3. Your email address may not be so private

Anyone can see a partially redacted version of your email address by simply clicking on "Forgot Your Password?" on Facebook's log in page, and entering your user name.

People on your friends list can probably see your email address, even if you haven't disclosed it on your profile. You can find your friends' email addresses by pairing your Facebook account with a Yahoo email address, logging into Yahoo email and choosing to import contacts from your Facebook account.

These privacy concerns can be solved by checking your Facebook privacy settings and by using a unique, non-guessable email address to sign up.

4. Your friends list may not be so private

You may have made your friends list private, but people can still see who has liked your pictures. Many of your friends will have clicked like on some of the photos you have uploaded and a list of these people can be obtained by searching for "People who like pictures posted by" you.

Additionally, anyone who "friend requests" you will see your private friends suggested to them as "people you may know", whether or not you accept their friend request.

5. Different profiles have access to different searches

Facebook's search is really inconsistent. Different accounts have different search features, so if you have no luck using your own account, try using an old account or getting a friend to search for you.

You may get a much better search facility if you change your language settings to "English (US)" instead of your native tongue.

9. Searching for photo comments & likes

Using Facebook Graph searches, people can see a list of the publically viewable photos you have clicked "like" on and read the comments you posted.

Once again, this involves looking up a user ID and pasting it into a web address. Let's use Mark Zuckerberg's again. If you look in his page's html code, you will see that Mark's Facebook ID is the number 4.

A Graph search address to find the photos he likes, would be:

A Graph search address to find the photos he has commented on, would be:

https://www.facebook.com/search/4/photos-commented

You can also do a Graph search to find the photos he has been tagged in:

https://www.facebook.com/search/4/photos-tagged

10. Photos can be traced back to Facebook accounts

Images on webpages have their own file names. Some might be named after the subject (e.g. "centralpark.gif"). Some might have been automatically named by the camera that took the photo (e.g. "dsc_1234.jpg"). Others will have been named by a website's database.

You can see an image's name by simply right-clicking on it and saving it onto your computer.

If you upload an image to Facebook or Instagram, its name will be changed to something really long and complicated - usually consisting of 3 bunches of numbers, separated by underscores and finishing with n.jpg. So if you ever see an image named like that it's probably spent some of its life on Facebook.

The second bunch of numbers in this file name, relates to the Facebook account the image was uploaded to.

The photo above belongs to the photographic artist, Marc Blackie. If you copy and paste the second bunch of numbers into a Facebook.com web address, you'll find the image in Marc's Facebook photo album.

http://www.facebook.com/10152728722695485

Of course, that's no guarantee of the subject and ownership of any photo, but this technique can provide useful clues in an investigation.

11. You can search the net by profile picture

People often use the same profile pictures on various websites and social networks, so it's often handy see see where else they pop up.

You can do this by uploading saved pictures to services like Tineye and Google Images (click on the little camera icon in the search box).

If you are using Chrome, you simply have to right-click on the photo and choose "search Google for this image".

Google will then show you addresses of other pages where your chosen image appears, e.g. Twitter accounts, blogs and personal web sites.

12. You can tell Google Images to only look on Facebook

You can restrict a Google Image search to Facebook page, by adding site:facebook.com to the search box.

This also works with Google's reverse image search feature, as does the date range feature found by clicking on "search tools".

13. You can give yourself extra search powers

If you are prepared to do a little bit of research and some cutting and pasting, you can make your searches more powerful and accurate.

Facebook introduced its new search back in December 2014, but it has many problems, not least its inability to specify search subjects. You can search for mutual friends of Dave Jones & John Smith, but you can't specify which Dave Jones or John Smith you are interested in.

The previous Facebook "Graph Search", allows you to specify people and pages in your search, and a lot more beyond. You can still use it. Find out how by visiting this guide.  See also graph.tips.

Facebook's old Graph search allows you to specify people and pages in your search

14. There are many ways to research topics in Facebook

The Facebook Search Box

You can use search box, but it tends to give you stuff your friends posted. It's a little bit fuzzy and not much use for journalists, but it might me useful.

If you are after a page, rather than a person, you need to specify this in your search. For example, a simple search for "Steve McQueen" will find you pages about the 1960's U.S. icon, rather than the British director of 12 Years a Slave.

Instead, search specifically for the phrase pages named "Steve McQueen". The results are radically better and include the British director as well as the U.S. actor.

You may find this approach helps with all of your searches for people, places, photos, events etc.

You can also search through your own Facebook posts by searching "my posts about" whatever it is you need to find.

Google
You can search Facebook via google, using the syntax site:facebook.com. You can specify which words should be in the title of the Facebook page by using intitle: followed by the word. For example, Facebook pages that are about Interpol, but mention Sweden.

Graph search for Topics by ID
You can use find a reasonable cross section of posts on a subject by firstly finding its page's Facebook ID code....

The BBC News page has the code 228735667216

...and then adding this code to the address http://www.facebook.com/topic/ - for example, posts about BBC News.

Find Hashtagged Posts

You can search for posts that have been hashtagged in Facebook by simply adding the tagged word to the end of this web address:

https://www.facebook.com/hashtag/

posts hashtagged with #clarkson

Interestingly, you get different results by typing posts with the hashtag "clarkson" into Facebook's search box, so it's worth trying that as well.

15. You should save save pages and archive your own stuff

Facebook accounts often disappear within hours of someone making the news. People make their walls and photos private without warning. If you are investigating someone on Facebook, remember to save copies their pages - anything poignant to the piece and especially friends lists.

You can do this, crudely, by highlighting text, copying and pasting into a word document, but it is much better to save each page on the profile as an individual web page. You can do this in your browser by typing control +s and choosing a place to save the files on your computer.

Once this is done, it's a good idea to open it up again and check that the page has saved properly. Some browsers save the wrong page. If you have problems, switch to a different browser.

You may also want to save all the photos, posts and friends lists from your own Facebook account. This is easily done. Go to your settings, click on "Download a copy of your Facebook Data" and follow the instructions.

If you have any Facebook tricks, tips or secrets to share, please get in touch.