It’s more likely than you think!
While some people are very frustrated by the occasional spamminess of Twitter -> Facebook posting, and others posit that Facebook will eventually kill Twitter because the “conversation moves there”, I just like being able to update both places at once and don’t really care to make predictions either way.
Instead I want to post a quick field guide to Twitter for Facebook users. Not because they should particularly go ahead and sign up, but to make clearer what all the @this and #that’s crapping up their news feeds are. Because they do tend to open dialogs and conversations, but can be confusing sometimes too – I definitely think about how something will work on my FB feed before posting to Twitter.