Twitter said its number of monthly users fell in the second quarter, and will likely continue to drop as it shuts down fake accounts, sending the company’s stock tumbling 16% in early morning trading.

The number of monthly active users dropped to 335 million in the quarter, a decrease of more than 3 million in the quarter. The company said its decision to remove spammy and suspicious accounts, while dragging down its metrics, were important to improving the quality of conversation on the social media platform.

The company said it is now challenging more than 9 million questionable accounts a week, up from 6.4 million last December. Technology improvements have resulted in removing twice the number of accounts that violate Twitter’s spam policies.

“We will continue to work hard to improve the health of the platform, providing updates on our progress at least quarterly, and prioritizing health efforts regardless of the near-term impact on metrics, as we believe the best driver of long-term growth of Twitter as a daily utility is a healthy conversation,” Twitter said in its letter to shareholders.

Investors showed concern, sending the stock down 16% to $36 as trading began this morning. The report follows Facebook’s historic plunge yesterday, in which it lost $119.1 billion in market value — the largest single day loss ever for a publicly traded company.

Despite the decline in monthly users, Twitter posted a quarterly profit of $100 million compared to a loss of $116 million a yea ago. Its per-share earnings of 17 cents met consensus estimates. The company’s reported revenue of $711 million, up 25% from a year ago.