If you have recently logged on to your Facebook account, you must have noticed this Bar on the right hand column of your Facebook page showing you some profiles which are already friends with many of those which you are friends with. Cutting down the crap, its just an enhancement of the Mutual friends feature on Orkut. Lets have an insight into what must have gone into this app from Facebook. Building apps on facebook publicly is never a problem, thanks to Facebook Markup Language (FBML) and the Facebook Query Language (FQL), which is much similar to the Structured Query Language.
What happens in the background is most probably this: First a random no., say x, is generated and then Using the FQL, your friend`s database is queried for that no. of friends. Now again using the FQL, a query is sent for those friends who are on your friends` friend list but not on yours. This thing is done for the x no. of times with the remaining friends on your list. Now if one particular profile, say with ID#AB45456 shows up every time on say more than 2 of your friends` friend list, then FB suggests you to check the profile of ID#AB45456 in case you both know each other and fate has kept you both apart till now on facebook.
This, you may call to be a WEB 2.5 deliverable, as in suggestion based APIs are on their high nowadays. Socializing Online wouldn`t have been this better without these crispy features. I would like you to test one small facebook app which I have written, just to show you how simple it is to get an app up and running on FaceBook. Cheers!!