We all know that advertising is very important to Facebook, but have you ever wondered how important Facebook is to advertising? Very, very important, apparently.

The social networking giant accounted for 346 billion display ad impressions in the U.S. for the first quarter of 2011, or 31.2 percent of all impressions displayed in the quarter, according to comScore. To restate, that means Facebook last quarter was responsible for nearly one in three ad impressions.

Runners-up don’t even approach that kind of reach.

Ranked just below Facebook are Yahoo! Sites with 112 billion impressions (10.1 percent share), Microsoft Sites with 54 billion impressions (4.8 percent), and AOL, Inc. with 33 billion impressions (3.0 percent share). If you combine all three and add up the market share of the rest of the top ten list, it only comes to 25.6 percent, or still several points below Facebook.

What makes it all the more impressive is the fact that one year ago, in Q1 2010, Facebook only held 16.2 percent of the market. It has nearly doubled that number.

The truth is that we shouldn’t really be surprised by these new statistics. Signs of the social advertising boom are everywhere.

Social media advertising revenues are expected to quadruple from $2.1 billion in 2010 to $8.3 billion in 2015, representing annual growth of 31.6 percent, according to BIA/Kelsey’s U.S. Local Media Annual Forecast published earlier this week. And that only adds fuel to the fire of Facebook’s earnings, which are expected to top $2 billion this year.

comScore also reported on the top display advertisers, though there is a far less divide there than amongst publishers.

AT&T ranked as the top advertiser in the U.S. in Q1 2011 with 19.5 billion impressions, or 1.8 percent of the market. Ranking fifth was AT&T’s top competitor Verizon, with 9.7 billion impressions for 0.9 percent market share.

Filling out the rest of the top ten are mostly Web-based companies in a wide variety of sectors, including Experian (credit reports) with 16.6 billion impressions for 1.5 percent share, ScottTrade (online investing) with 11.2 billion impressions for 1.0 percent share and Intuit (small business accounting) with 11.0 billion impressions for 1.0 percent share. Notably, Netflix and Groupon also made the list.