Twitter is a really easy service to underestimate. If Facebook could wipe out a competitor that easily don't you think they would have done this already?
Okay, I will prove my point using what you've just said: if people are importing their Twitter feeds into Facebook, instead of letting Facebook status serve as their alert beacon for friends, what do you think that says about the user value of Facebook status updates?
For some people at least, Twitter beats Facebook simply by only doing statuses and not exposing you to all the other crap.
With Twitter, I don't have to waste my time ignoring the latest Knighthood Invitation or the Group Created to Get Enough People in the Group to Advance Cause X w/ No Substantive Effort Invitation.
That is why Facebook can't easily kill Twitter.