> because it generates different content for almost every pageview. This means much less possibilities for caching.
That implies they are caching very little. I'm comfortable in saying that I'd be shocked if your homepage on Facebook wasn't 90%+ in cache, if not 100%. A site like Facebook wouldn't survive without caching.
I am not sure how you define those percentages, but Facebook's homepage is regenerated on every pageview. Storing the entire homepage html in cache for every user would not work, because every time someone posts something you'd need to regenerate the homepage for all their friends (# of times someone posts something * avg # of friends > # pageviews homepage).
Sorry, I was referring to the data, not the HTML output. Even still, you could cache components of that page. Your feed might update often, but their are many things that don't change on ever page load, and they can be cached for a long time.
That implies they are caching very little. I'm comfortable in saying that I'd be shocked if your homepage on Facebook wasn't 90%+ in cache, if not 100%. A site like Facebook wouldn't survive without caching.