Facebook has more worldwide visitors than its main rival, MySpace, but the social network has struggled to cash in on its seemingly unlimited advertising potential. Enter Facebook's new engagement ads, which the company hopes will be a success with both its members and advertisers.

Engagement ads appear on the Facebook homepage when a user logs in, asking a user to RSVP for an event, such as a television show premier. If a user responds to the action, Facebook then notifies the user's friends via the news feed feature.

The engagement ads, which Facebook began experimenting with in August, cost more than other display ads on the social network.

The new ad strategy comes amidst questions about whether social networks, and Facebook in particular, can ever become viable marketing tools. Facebook has close to 160 million global users, but its share of U.S. display ad spending is just 1.1 percent, proving that marketers simply aren't spending on the social network. MySpace's share of display ad spending is 15.9 percent, according to comScore.

"I haven't heard of anyone purchasing something off an ad on Facebook," Angie Tulgetske, VP of RE/MAX Preferred Choice Properties, told The Wall Street Journal. "I wouldn't think any of my marketing dollars would be spent advantageously there."

Tulgetske may have a point. Users have complained that Facebook fails to deliver relevant ads, and according to Forrester Research analyst Jeremiah Owyang, less than 1 percent of users click on Facebook ads.