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.

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