As social networks struggle to monetize, targeted display ads have become synonymous with the social media experience. But which network serves ads the best?

According to a reader poll at Mashable, the winner is Facebook, which delivers the "most accurate, interesting and engaging" ads.

Facebook, which uses its own Facebook Ads platform, received more than three times as many votes as second place finisher Friendster. In fact, Facebook received more votes than the second, third and fourth place social networks combined.

But that doesn't mean social network members actually enjoy these ads. Mashable readers cast the most votes for "They're all equally horrible," and one commenter recommended the post's title be changed to "Facebook has the least terrible ads."

So what makes Facebook the ad-serving king? One word: relevance.

"[T]his is only a personal observation, but obviously one that is shared by many of our readers -- [Facebook's] ads seem to be much more relevant than the ads on other social networks," wrote blogger Stan Schroeder.

A report by Pali Analyst Richard Greenfield backs up Schroeder's opinion. In his research, Greenfield found that the ads served by Google on MySpace are often completely irrelevant.