Top social game developer Zynga, quiz-maker Pangea Media and casual game site GameVance were three of the top advertisers by impressions on social networks this past June, mostly MySpace and Facebook, according to a comScore report on social network advertising published yesterday.

While still dwarfed by some major brands, gaming companies are getting more impressions than some and doing an effective job of drawing people to their games.

Zynga had 484 million impressions, for 96.8 percent of its total impressions on the web, with 40 million unique visitors to its applications as a result. Zynga notably spent a couple million dollars to advertise its hit new game, FarmVille, starting in June. Its spending on Facebook’s own advertising service has been successful enough that we hear many other developers are beginning to use it as well. A metrics-driven company, Zynga has apparently honed its ability to target users through ads. It has found these ads cost-effective because they can make more money from these games — possibly above $200 million by the end of this year, I’ve heard from various source. In general, as competition among games continues to heat up, social network advertising has become an important channel for developers to gain new users.

Pangea had 572 million impressions, 89.6 percent of the company’s total online ad impressions, and 33.1 million unique visitors. GameVance had 450 million impressions, only 33.9 percent of its total impressions on the web, with 34.5 million unique visitors coming to its site as a result.

In terms of larger brands, carrier AT&T had 2.07 billion impressions for 29.5 percent of its total, but only 87.1 million unique visitors. Verizon had only 435 million impressions — fewer than the three gaming companies — for a small 10.5 percent of its online totals, but it received 54.1 million visitors as a result.

Note, however, that it’s not clear how fully comScore is able to measure impressions on Facebook; comScore numbers should be taken more as one view of social network spending rather than the a complete picture.

And one last thought: The three gaming companies, and likely many others, are mostly advertising on social networks now, because social networks are the easiest place to find social network users. Zynga and other companies will likely use some of their revenues to try to find more users in other channels as well, through using behavioral targeting, click-trails and other data to figure out where on the web they can reach social network users.