For the second day in a row, Twitter stock was down.

It closed the day down 3.74% at $30.66 per share. That's on top of Tuesday's plunge of nearly 18%.

Part of the reason why Twitter fell so much this week is because the employee lockup period expired Tuesday. Many employees likely cashed in their stock. But it's important to note Twitter's founders and biggest stakeholders have said they don't plan to cash in.

Twitter's tumble began last week when it reported that its monthly active user growth is slowing. It has 255 million monthly active users, which is smaller than Facebook (1.28 billion), LinkedIn (300 million), and WhatsApp (500 million).

Still, Twitter's financials look pretty good. And because Twitter bought the mobile ad company MoPub last year, it has a nice safety net in its mobile ad network.

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