A while ago I wrote about one difference between Twitter and Facebook: The way you select or make contacts on each of the services.
Another big difference is what you can write in a posting. On Twitter it's 140 characters maximum, but on Facebook it's 420, three times as many.
How many posts (status updates) are that long? Only a small percentage to my knowledge. In fact lots of the posts on Facebook would fit within a Twitter message. But there's more than status updates on Facebook. There's the Notes section too with much more space for expression.
Then there's Facebook's Like-button, which applies to single posts as well as for whole Facebook pages and groups. This is a great feature. It's so great that Google has copied it to its search results and more places in the hope of making its users interact more. You can also comment on posts, and sometimes it's so fast it's almost like chatting. BTW there's a separate chatting function too. And email.
Looking at Facebook and Twitter you notice that Facebook is a lot more comprehensive service. It's a universe. Twitter isn't. Within Facebook you have your wall and your friends' walls. You have the notes section and games and other applications that need Facebook's environment to function. You have events. You can have Facebook copy all your blogging onto your Facebook wall. You can do the same for other services like Bambuser (your private online TV station). You can be member of secret societies through a secret Facebook group. Due to all the above, I say Facebook is a universe.
Twitter does not have these aspirations, which lots of folk like. Twitter is for those who don't want to bond too much, and that includes many Facebook users. Twitter is somewhat more anonymous, while Facebook is anything but.
If you got this far: Thanks for reading! :-)