I'm sure this is going to come up repeatedly: what can they offer me from an eCommerce perspective that Amazon can't? Like most social network benefits, I think friend recommendations, which I can and do get in real life, are overrated and useless for the majority of my purchases.
The gifting idea is quaint. But to be full-on eCommerce site, well that's yesterday's news.
This is going to turn Facebook into the Amazon for gifts. They capture you exactly at the right point, can (eventually) give pitch-perfect recommendations, and can get the checkout flow down to be completely painless. If they execute on this well (and I expect they will) I would guess most gift purchases will be happening on Facebook in a few years.
The gifting idea is quaint. But to be full-on eCommerce site, well that's yesterday's news.