Yes, right Facebook is best among above and now Facebook is launches Graph search tools for Internet users that you can easily find people, friends, places, interest etc.
In my views facebook is the most useful among the all social sites.It provides numerous features which increases the users day by day.It is being used by all types of persons in different different ways.
There are many but if i choose from them, then -facebook.com -youtube.com -qoura.com (ans/ques collect knowledge) -livemocha.com (languages learn)
I would personally say Twitter or Google+. I have had the best success in marketing on them more so than I have with the others so those two are my choices.
i think facebook has lost lot of their fans & user to twitter, the thing is social networks fades depending on the new one,