Facebook served 16% of all display ads in Q1 of this year, according to comScore, making it the largest online display ad publisher in the US—handily beating #2 Yahoo, reports ClickZ.

Facebook’s growth is impressive whether compared to its Q4 numbers or Yahoo’s numbers. In Q1, Yahoo’s properties saw 132B impressions (12.1% of all online display ad impressions). In Q4, Facebook served about 115B impressions. In Q1 of this year, Facebook served a whopping 176B impressions: a 53% increase over its previous quarter. (Yahoo saw a slight decrease from Q4: down from 140B impressions, or 6%, and all other major ad players also saw declines.)

Last year in Q1, Facebook served 70B ad impressions, which means they increased their ad impressions by over 150% YOY.

And what do all these numbers mean? Well, first of all, Q1 had more than one trillion online display ad impressions. comScore CEO Andrew Lipsman also pointed out that the ad growth correlates with the number of page views they’re seeing for Facebook:

We’ve seen increases in the number of visitors to the site, and also increased intensity of use which results in more pageviews. The increase in pageviews has been pretty consistent with the increase in ad impressions.

However, these numbers don’t mean that Facebook is making more than Yahoo. CPM can obviously vary from site to site—and Facebook has some major changes coming for its CPM advertisers that will mean fewer clicks. It’s also important to note that these numbers don’t reflect Yahoo’s ad network, just impressions on Yahoo properties themselves.

What do you think? Are advertisers following users in a shift away from portals to social networks? Where is advertising more effective?