With LinkedIn filing to go public, we finally get a look at the company’s revenues. LinkedIn has a healthy mix of three businesses – paid subscriptions, advertising, and hiring solutions – all of which are growing snappily. But as Silicon Alley Insider points out a bit harshly, social media is still a relatively tough sell to advertisers. LinkedIn’s ad revenues were $51 million for the recent nine months. Likewise, recently public content farm Demand Media’s 9-month ad sales totaled $106 million, about $9 million of which it shared back to network partners. Researcher eMarketer thinks Twitter did $45 million for all of 2010, but it’s just guessing. It also estimates Facebook did $1.86 billion. Compare that estimate with Google’s 2010 actual of $28 billion. Still, Facebook may be approaching Yahoo‘s $5 billion, and Microsoft‘s approximately $2 billion – and Microsoft has never made consistent profits online.