As mentioned before, your goal is to be operating as an E3 MA (Meets All Expectations) by the end of your internship. Assuming the standard 12 week internship, you have 10 weeks to do this.
You will be working on production code, dealing with Meta's gigantic codebase. The exception is FBU (FB University, a "pre-intern" program) where you build an app/website from scratch in isolation.
Every week is crucial. A great/terrible week can make or break your internship.
Raw coding ability is the main thing you're judged on as a SWE intern (literally ~90% of your packet). The 2 main components here are velocity and quality, which means that your goal as a SWE intern is effectively just to write a lot of very high-quality code fast.
In a vacuum, target landing 3-4 diffs per week. Of course, this will vary based on org, stack, etc - Work with your intern manager (IM) to get a more accurate range. You can look at previously successful interns in the org to generate a baseline as well.
Write 1-2 Workplace posts as technical writing is super important at Meta. The easiest thing to do is write about your intern project as you hit major milestones, especially when you launch it.
You need a cast of people to vouch for you during calibrations. Target having 2+ peers on top of your IM (i.e. so 3+ people total).
You will likely work on an internal tool as it's free from XFN thrash and dependencies. Meta has a lot of respect for quality internal tools as well.