Snap Inc. said its quarterly loss nearly quadrupled amid slowing revenue and user growth, sapped by intensifying competition from larger rival Facebook Inc.

The parent company of messaging app Snapchat said Thursday that revenue more than doubled in the second quarter to $182 million from $71.8 million a year earlier. Yet that growth rate was about half what it was in the first quarter, and slower than investors expected for a company that says it still has ample room to grow. Snap’s second-quarter loss ballooned to $443 million...