Of course there are alternatives. Twitter is an alternative (not saying its a killer though). An alternative doesn't (and really shouldn't) have to contain all of the same features or work in the same way. It's just something that you use instead of. The Windows killer turned out to be "The Internet". Although it's a different beast, it moved Windows to the background. The Facebook killer might very well be a protocol for sharing things on the net. Or simply more sites adding their own social features. Or something no one has thought of yet.
I don't see Twitter as a competitor, for me they're too different and I don't see any problem using both of them parallel. Comparing Windows apps to web-apps; I think for the end-user they're conceptually very similar, just delivered differently. Is GMail that different from Outlook to the user? I think your average person will want something at least a little similar to Facebook; I don't think Twitter is that thing.
