Facebook appeals to people looking to reconnect with old friends and family members or find new friends online; the mashup of features like email, instant messaging, image and video sharing, etc. feels familiar, while Twitter is a bit harder to get your arms around at first.
Most people can very quickly grasp how to use Facebook to connect to friends and family, using it to share thoughts, images, etc. Like MySpace but more geared to adults than teens, Facebook is a social networking Portal; beckoning you not to leave but rather to stick around and communicate within the network.
Twitter on the other hand, encourages you grab ideals in byte-size chunks and use your updates as jumping off points to other places or just let others know what you’re up to at any given moment.