Twitter posted better-than-expected quarterly earnings on Tuesday, along with monthly average users that climbed 24% year over year and ad revenue that increased 129%. Shares of Twitter soared on the news. As a result, Brian Wieser, senior analyst at Pivotal Research cut his rating on Twitter to Sell from Hold. He explains that with the price rising in heavy volume in the after-market, the trading price was more than 15% away from what he believes is fair value. He breaks down his thoughts on the quarter, the key risks he sees to Twitter's valuation and why user growth is not nearly as important to him as it seems to be to others.