Easier and quicker than I thought. The interview had only two rounds, each with two LeetCode questions. They don't ask any behavioral questions at all. You can find questions online; they aren't super obscure.
You can find the questions on LeetCode.
The following metrics were computed from 546 interview experiences for the Meta Software Engineer (Internship) role in United States.
Meta's interview process for their Software Engineer (Internship) roles in the United States is very selective, failing most engineers who go through it.
Candidates reported having very good feelings for Meta's Software Engineer (Internship) interview process in United States.