Twitter and Facebook are two different types of sites, so it's not going to have the same userbase. Seeing as Facebook is so popular it's not surprising there are more from AH there.
Personally I use Twitter as I only want updates from a few companies, that I'll admit I'm too lazy to get from their sites, and I dislike Facebook. Facebook was lit up like the Vegas Strip last time I used it, even with NoScripts, and I don't need updates from everyone and their 8million games, anyone I know on Facbook I see everyday anyways. It's always going to be a personal choice on what site a person uses, which is likely why HTC uses both. You can't make people use site A if they prefer site B.