Not only that, but more importantly, on Twitter you can follow pretty much whoever you like without a required acknowledgment from the followed person/organization. To be friends with someone in FB, both parties have to agree, which pretty much sets them apart niche-wise.
Facebook: full-fledged social network. Twitter: simple, quick-posting environment.
There IS a big difference between these two sites, Facebook won't kill twitter, we can all go to sleep calmly now :)
Edit: just for reference, http://news.ycombinator.com/item?id=418151