QUOTE(sourabhmac @ Jun 5 2014, 05:06 AM)

Which strategy use Facebook for ranking and why they don't use seo services. what is reason of Facebook ranking? please explain it in brief.

They deliver quality services that people wants.
They have huge traffic too.
And their user create back links to Facebook when they use it and spread links all over on their profile of different accounts.
They have huge content on their site, of course posted by users.
Facebook have backlinks from high brand and reputed sites. They passes their link juice too.