At 4:45 p.m. on Thursday, the Twitter account for Mashable—one of the earliest movers into the now endless world of social media news sites—sent out its 60th tweet for the day.

The tweet itself wasn't particularly interesting, but what happened next was a small window into one of the biggest challenges Twitter will face as it seeks to convince investors that its more than 215 million users are one of the web's most lucrative—and undeveloped—advertising audiences.

Within minutes, hundreds of other Twitter accounts reposted the Mashable tweet word for word—more than 400 of them within the first 15 minutes. Scrolling through the never-ending list of those copycats is like a deep dive into the Twitter netherworld, one that regular users would seldom find themselves in.

Beyond the Biebers and Mashables, the CNNs and Kardashians, is a Twitter world populated by millions of accounts of questionable legitimacy.

They range from entirely robotic (and often incomprehensible) spammers to more cleverly programmed accounts spitting out tweets designed to find their way into the occasional search results or discussion thread.