Twitter shares fell more than 20 percent Friday after the company reported a decline in monthly active users and weak guidance.

Twitter reported second-quarter earnings before the bell on Friday:

Earnings per share: 17 cents vs. 17 cents, according to a Thomson Reuters consensus estimate

Revenue: $711 million vs. $696.2 million, according to a Thomson Reuters consensus estimate

Monthly active users (MAUs): 335 million vs. 338.5 million, according to StreetAccount and FactSet estimate

Shares opened down 14 percent. Today's losses mark the worst single day percentage drop since 2014 for the company, and the second worst in Twitter's history.

The company issued weak guidance as well, with adjusted EBITDA between $215 million and $235 million for the third quarter. The company expects stock based compensation expenses to be in a range of $300 million to $350 million for the full year, down from a range of $350 million to $450 million previously expected. Capital expenditures will be in a range of $450 million to $500 million, up from a previous forecast $375 million to $450 million.

For the last quarter, Twitter reported 336 million monthly active users. The platform blamed not moving to paid SMS carrier relationships in certain markets where users have better access to Twitter or Twitter Lite, making changes to improve the "health" of the platform and some impact from GDPR, a set of regulations in the European Union intended to protect consumer data. In total, Twitter estimates about 3 million accounts were affected by these three reasons.

Twitter removed about 70 million accounts in May and June, but Twitter chief financial officer Ned Segal said most of those were not included in its reported metrics because they were not active on the platform for 30 days or more.

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The company also recently purged fake accounts, but those changes occurred after the close of the second quarter, so it didn't affect MAUs in this report. Twitter warned MAUs could go down even more next quarter.

"As a result of our health work, decisions not to renew or move to paid SMS carrier relationships in certain markets, and our decision to allocate resources towards GDPR and health, MAU could decline on a sequential basis in Q3," it said in its shareholder letter. "Based on our current level of visibility, we expect the decline to be mid-single-digit millions of MAU."

Daily active users grew 11 percent, but the company did not break out the exact number.

Twitter revenue grew 24 percent year-over-year, with strong advertising gains. Advertising revenue was at $601 million, an increase of 23 percent year over year. The company also grew its data licensing and other revenue business, which was up 29 percent year over year.

Correction: A previous version of this story swapped the full year capital expenditure projections. Twitter is forecasting $450 million to $500 million now, but previously forecast $375 million to $450 million last quarter.