I can't comment on Facebook but at Google I see plenty of people in their mid 30s and beyond. I joined in my early 40s. Yeah, we hire a lot of new grads / younger folks (I interview a lot of new grads) but I haven't noticed any overt ageism in the groups I've been in. Plus, the people making the hire decision don't see the candidate anyways. You should/would most likely get called out by the hiring groups if you added in color/vague "cultural fit" references.
Well, that's a huge company with many different types of engineers, and in business a long time (relatively). I would say the ageism is more likely at small startups that happen to have mostly young (in their 20s) engineers.
That's likely to be self-selecting, isn't it? I know as a 37 year old engineer with 2 kids I value stability and a reliable salary over excitement/'changing the world'/foosball tables etc.
This is just my experience, though.