The shares, which have risen as much as 23% this year, fell to $201.40 in trading after the bell.
It was the company’s first full quarter following the Cambridge Analytica privacy scandal.
The firm said it had 2.23 billion monthly active users at the end of June, up 11% on June 2017, the slowest growth in more than two years.
Analysts attributed the user growth shortfall largely to European privacy rules that went into effect in May.
The Cambridge Analytica scandal prompted several apologies from chief executive Mark Zuckerberg and generated calls for users to desert Facebook, which has grown strongly since launching as a public company in 2012.
The company had warned investors to expect a surge in costs due to efforts needed to address concerns about Facebook’s inadequate handling of users’ privacy.
Total expenses in the second quarter surged to $7.4bn (£5.6bn), up 50% from a year ago. However, ad sales seemed unaffected by the scandal, rising 42% in the quarter to $13bn (£9.9bn).
Net income attributable to Facebook shareholders rose to $5.11bn, or $1.74 per share, in the second quarter ending 30 June, up from $3.89bn, or $1.32 per share, a year earlier.
Total revenue for the quarter rose 42% to $13.23bn, below analysts’ estimates of $13.36bn