Public online matter regarded as PUBLISHED MATTER; in IPC itself.
ipc 501. Printing or engraving matter known to be defamatory
Whoever prints or engraves any matter, knowing or having good reason to believe that such matter is defamatory of any person, shall be punished with simple imprisonment for a term which may extend to two years, or with fine, or with both.
check more for IPC here http://mynation.net/ipc.htm
and breif of Cyber law http://mynation.net/cyberlaw.htm
and more on defamation here http://mynation.net/defame.htm and what is not defamation.
If anyone published anything wrong and you can prove it, you can submit it to court, you can get user details from said sites to charge them with cyber law too.