Losses in social media stocks since Yellen's remarks have been biggest in companies that were already out of favor, like daily-deal sharing site Groupon and former social-gaming leader Zynga. Both missed second-quarter forecasts and were punished by the market, extending Zynga's loss since early 2012 to 82 percent and Groupon's drop from its 2011 peak to 81 percent.
Some economists praised Yellen for paying attention to even a small hotspot of speculation, even as they downplayed the risk. The social media sector, especially without the Big Three of Facebook, LinkedIn and Twitter, is too small to become a systemic problem, so Yellen probably "fingered these stocks to show that policymakers are carefully monitoring for potential bubbles,'' Moody's Analytics chief economist Mark Zandi said.
Others were less kind.
"It's a lot easier to place blame on investors for searching for returns in high-growth companies than for the Fed to criticize itself for the distortions it has made in keeping monetary policy so loose for so long," said Kathleen Smith, chairman and co-founder at Renaissance Capital. The Renaissance IPO ETF—which has Twitter as its No. 1 holding—is flat over the past month.
Read MoreTime to go to cash?
The month since Yellen spoke proves two things: Social media is growing fast enough in general that bubble talk can be misplaced, and investors can still get burned if they don't pick stocks carefully.
"Over time, you might not be able to make a living without very closely vetting the fundamentals,'' Spencer said.
—By Tim Mullaney, special to CNBC.com