I agree... never meant to imply that Facebook had fewer hits/etc than a banking site in raw numbers, just that the ratio of DB reads (not considered a transaction, very easy to cache, etc) to DB writes, makes the site much more read-only than write-only.
Facebook has the ability to do a TON of edge caching, with very few (relatively speaking) operations having to go to the database to perform an actual write operation.
It's the DB writes that really kill performance and limit your caching strategies and gains.
My guess is apps hosted on Heroku (like the majority of the internet applications) are a lot like Facebook and little like online banking: they don't really need reliable transactions, they need to be flexible, scalable and cost-effective.
FaceBook's actually one of the more dynamic sites out there. I'd bet that the average FaceBook user makes many more updates to their FaceBook profile than they make financial transactions.
The real difference in requirements is that FaceBook can - and does - drop updates on the floor. If your friend throws a sheep at you and you don't get it on your news feed - oh well, FaceBook never claims the news feed is exhaustive anyway. But if somebody writes you a check for $10,000 and they see that you cashed it and you don't see it in your account, that's a problem.