Pageflakes has taken the next step from being just a start page, by
adding social networking functionality. Netvibes hasn't yet gone that
far, but I suspect it's coming soon.
However there's still a lot of work to be done by both start pages
to catch up to Facebook or even MySpace - and that's purely down to
number of third party widgets and of course number of users on the
network. So while I agree with Josh that start pages will challenge the
existing social networks - it's almost evolutionary for them to become
social networks - it will need a big influx of developers and users
before they're truly competitive in that space.