Simon, I am a Canadian founder. I got my first job offer in the US (San Francisco) at 25 years old. I took it right away. It was not FAANG, though I was interviewed there many times: not once was I given an offer. My degree was in Biochemistry, not CS. I probably would have been over the moon to get an offer from FB. So I can understand how you feel.
However; I think you should consider finishing your degree. The job offer from Facebook will always be around -- or some equivalent. If you can get that at 20/21 you can get it at 21/22. The only time where I'd recommend somebody drop out is if they're committed to a project that is either clearly a business or has the potential to be one and they're pursuing entrepreneurship. I know a number of founders that have pursued this route and are quite successful. That said; college is really a once-in-a-lifetime experience. I wouldn't trade my time in college for anything, even though coming to Silicon Valley at 25 years old made me feel ancient. If you focus your last year on personal growth, hacking, friends, your network, building relationships - both at McGill and around Silicon Valley - you'll be even more set up to succeed post-graduation.
Re: the FB offer. The ugly truth you're glossing over is that working at FAANG can -- for many -- ultimately be just as boring as coursework in college. If you want to get into startups and like the energy surrounding them, my recommendation is use your last year of college to (1) kick college's ass and get that degree so immigration is easier and (2) start a badass side project.
That's my $0.02 but YMMV. Shoot me a DM on Twitter if you feel like chatting.