Most of my friends on Goodreads came from other social networks. But I don't see that as a problem. For a start, it reinforces relationships with people from elsewhere, which isn't a bad thing. It shows them my reading and reviewing side (rather than my random rambling side from Twitter).
And the real strength of Goodreads is its giant database of books, authors and reviews. It pays to make sure your profile is filled out properly, your blog is linked in and your book data is correct. If you review things, you'll probably get a few visits to your profile from that alone.
Aha! Another reason to review.
Good point about the profile, too. Someone at Viking pointed out that it was pretty sparse, so I went back and elaborated.
Now that I'm more out in the cyberworld, I'm starting to meet the same people on multiple sites, as you point out. But for me it's never fully real till I press the flesh.