As a 53-year-old Facebook employee, I find the part about Facebook doing this for their own hiring a bit odd. To the extent that anyone has ever reacted to my age with anything other than mild surprise, people generally seem quite glad to see something besides another fresh 20-something face and to hear the voice of long experience. Then again, I didn't come in via a Facebook ad. Maybe companies use different methods…
Can you speak to ageism at facebook? You must be quite rare there. Anecdotally speaking, whenever I visit the campus it definitely is a majority of 20-somethings walking around (or at least not many devs over the age of 40. I believe there are proportionally a lot more at Google).
I can share my own experiences, but I can't and probably shouldn't try to address it generally. I'm not just a sample of one, but I'm well aware that I'm an outlier in a couple of ways besides my age. So take all this with a grain of salt.
Yes, you're right, HQ totally feels like a college campus. Some of that's deliberate, from the architecture to the cafes and other amenities. I'm sure it feels comfortably familiar to the high percentage of people you see walking around for whom college is clearly a very fresh memory (or current "normal" during intern season). To me it feels a bit weird, but still quite pleasant.
I've honestly not seen any evidence of overt ageism since I've been here. There are other people my age, though even after a year I can probably still count the ones I've actually met on one hand. AFAICT nobody believes being older makes one less fit to be there or in a specific role. They sure don't believe it makes one more fit either, but that's OK because TBH that's something I'd become a bit too comfortable with and that's my own fault. If there's ageism, it's a very soft kind - more to do with comfort levels than "harder" boundaries or opposition. Some examples...
* Open-plan offices are objectively, provably even harder on older people than younger ones, because of hearing and focus/attention issues utterly unrelated to the actual work.
* Lots of people complain about the artificiality and induced stress of coding interviews. Well, it doesn't get better with age. As awkward as it might be to stumble on these when you're still young, imagine how it must feel when you're being tested on something you've been doing for 20+ years and the person judging you is half your age. The one truly negative age-related experience I've had (not in the interview) has been people who look down their noses at "old fashioned" code because they themselves have never known a world in which their favorite libraries either didn't exist or were buggy as hell.
* Being able to work from home is much more important for older people who are more likely to have families, minor medical issues (including other family members'), etc. Therefore a general antipathy toward letting people work from home is in effect discrimination against older workers.
So it's not like anyone has ever actively tried to stop or discourage me from working here, but there are ways in which we could be more welcoming nonetheless. Does that answer your question?