Would Letting Meta Know Of Another Offer Affect Their Hiring Decision?

Would you be ok joining that other company in February? What is the deadline to accept/reject that offer?

If you are genuinely ok with joining the company, you should absolutely tell Meta. Indicating that you have another offer shows that you have value in the market, and it will speed up the decision for you. (it's a way of gaining leverage since you can safely walk away now.) Say something like:

I would love to join Meta but I have an offer at another company which is expiring at X. I wanted to check back with you if it's possible to receive a decision from Meta in the next week as I make my decision.

You're right that one negative outcome from this is that Meta may simply reject you. But I don't view it as a much of a downside -- you're probably speeding up the eventual decision.

Would Letting Meta Know Of Another Offer Affect Their Hiring Decision?

Yes, it will make them want you more, especially if the other company is a competing Big Tech company.

Tech companies aren't charities. They're not going to reject a great candidate because they already have an offer and will be okay for the struggling candidate who only has Meta remaining as a potential opportunity. Their goal isn't to maximize employment among the labor pool, it's to get the best possible people (this is especially the case for Meta and FAANG overall).

By showing a competing offer, you are signaling to them that another smart group of tech people have blessed you with a mark of competence, which reduces the risk of you being a weak hire. You are statistically safer for Meta to hire as the engineers with competing offers are better overall like 99.9% of the time. I know a couple engineers who can get 5+ FAANG offers like it's nothing (even in this economy), and they're all genuinely brilliant.

If there's some tactical limitation like headcount not being available but they still genuinely want you (i.e. you got a "Strong Yes" on most, if not all, rounds), they will try to counter-negotiate by asking you to wait a little longer or something (or maybe even extend an offer outright with some very late join date just assuming things will be worked out by then). If they were erring on rejection, sharing the competing offer will almost certainly not change the outcome. You'll just potentially get rejected now instead of inevitably later.

Another thing I'll say is that Meta E3 hiring is insanely competitive, especially if you aren't a returning intern. I wouldn't hold out on Meta, especially if you need to make some sacrifices like rejecting an existing quality offer in hopes Meta pans out later on.