Everybody knows social media firms are growing like gangbusters. Facebook, for instance, recently surpassed 350 million users, 16 months after reaching 100 million users.

How much money the Web start-ups are making is less clear — though industry-watchers were treated to a few tantalizing revelations this week.

Facebook is within striking distance of a $1 billion annual revenue run rate, according to TBI Research analyst Rory Maher.

FB2That would be a dramatic increase from the $500 million in 2009 revenue that Facebook board member Mark Andreessen touted back in July.

In a report released Tuesday Wednesday, Maher, whose research firm is own by blog publisher The Business Insider, said Facebook’s revenue growth is coming as advertisers increasingly pour marketing dollars into social media web sites.