Facebook is hitting majority adoption while Twitter is stealing the early adopters. They are simply in separate places on the adoption curve.
1. Twitter has nothing to do with early adopters anymore...they were on freaking Oprah.
2. They've been around for 3.5 years, which is only a couple years less than Facebook, so it's not like they're this little upstart that's trying to unseat the ancient Facebook.
3. Facebook has way more retention and engagement / user.
4. Facebook does what Twitter does (status updates), and does it better, IMO.
5. My parents and aunts and uncles and grandparents all use Facebook. They find value in it, and it enriches their life. Twitter is useless to them.
6. Twitter now is mostly marketers and spammers, judging from the people trying to follow me.
7. Most people seem to talk on Twitter way more than they listen. Over time, this drives down engagement, which means that people have to talk louder and more often to get any attention.
8. Most importantly, Facebook has hundreds of millions in annual revenue, and is predicting cashflow positive in 2010. Twitter has yet to make a dollar (to my knowledge).
EDIT: Added a few more points
On the Oprah point: Oprah was a very recent event. Not sure it's had a long enough time to make an impact. And adoption of Twitter has gone up significantly since that spot.
Umm... :)
Regardless, it's been more than four months since that point, which is around 10% of Twitter's total life.
Twitter actually does it better -- but, because Twitter doesn't do anything else, everything on Twitter goes into the "status update" interface, which results in severe clutter. In short, Twitter's status update functionality is better, but the social effects on it are damaging enough to make Facebook actually better suited to status updating in practice.
It's sort of an academic quibble, I guess, but sometimes those are the most fun.
Besides that I find this whole comparison ridiculous. The friend vs follower dichotomy pretty much puts the two services in different categories. In theory some third party service could overlay some Facebook-like functionality on top of Twitter, but will such an arrangement have the usability and critical mass that Facebook has? Will Twitter be compelling to regular people as it becomes more and more dominated by marketing? Is the 1% of Facebook functionality that Twitter provides compelling enough to get the masses to switch?
Do you have anything to support his at all?