IMHO a lot of people stop with twitter because they don't get the right social circle going: they follow a bunch of people but don't gain much followers and who they gain aren't really engaging in any activity, so it feels as if they're posting text in a document they store on their own computer: no-one really sees it nor cares.

Twitter has the downside that it's really 'in the moment': stuff that's tweeted now is interesting now, not tomorrow. This gives a problem with people who open twitter occasionally: they're likely not on twitter when something happens IRL and everyone's tweeting about it. With Facebook this is different, as things aren't really 'in the moment', or at least aren't depending on that that much: it's perfectly fine to read someone's post 2 days after it's been posted.