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What: Shares of Twitter (NYSE:TWTR) sank today, down by 10% as of 3 p.m. ET, after the company received an analyst downgrade alongside a troubling data leak.

So what: Stifel Nicolaus downgraded its rating on Twitter from hold to sell, claiming that it should have had that rating "all along." Analyst Scott Devitt notes that Twitter's core product was never fully developed in a sustainable fashion. Meanwhile, Business Insider analyzed data using Twitter's application programming interface, or API, and found that aggregate tweets per day is steadily declining.

Now what: According to the third-party data leak, tweets per day peaked way back in August 2014 during the World Cup, but have steadily fallen in the months and years since. Twitter's official response via a spokesperson was that the data is simply "not correct," but Twitter did not provide any additional detail. At the same time, the fact that Twitter chose to stop reporting Timeline Views per user last year doesn't inspire a lot of confidence in user engagement. Stifel Nicolaus believes that the only way for Twitter to bounce back is if it can improve its product strategy and execution.

Evan Niu, CFA has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.