Facebook is the largest social network worldwide, but it's still woefully behind rival MySpace in the U.S. market, and it's not going to catch up anytime soon, according to a TechCrunch story.

According to a post by Michael Arrington, music could be the key factor that separates the two in the U.S. market. That's bad news for Facebook because MySpace has built a solid audience around music, becoming the de facto launching pad for bands, whether they are signed to major labels or budding hopefuls.

By contrast, Facebook doesn't have an audience built around music, nor does it have a strategy to build that audience, which means that at the current rate of closure it will take Facebook 18 years to catch MySpace in the U.S.

Of course, a lot can happen in 18 years, and one need only look back over the past 18 months to see how quickly Facebook has caught up with the News Corp.-owned titan. But if Facebook's audience is largely international, it may not be worth as much to U.S. advertisers, which the social network is counting on to help it monetize traffic. That fact could put a damper on Facebook's latest ad evolution, dubbed Engagement Ads, which is designed to insert major brands into the daily activities of Facebook users.