No, I do not think you are wrong. But I do think things are not as clear as you suggest — even looking backward.

Regarding technology's ability to enhance social connections … I think it can also alienate. Every neighbor in front of computer screen is a neihgbor not on the sidewalk, where the most meaningful connections are formed.

Brian

I think it's clear why some of your above examples thrived, and why other's haven't yet.

It's very much got to do with the marketing approaches taken.

For facebook it was owning one segment, the college market, that gave it a foothold into the greater market.

For Yahoo answers, I wouldn't say it's taken hold, but the limited success can be accounted for by sharing the power of yahoo itself.

For Your Street, they've got to get ahold of at least one market, like facebook did.

Am I wrong here?

And regarding social erosion, I'd say technological shifts have made it possible for us to function more easily without being closely connected to our neighbors. However new changes online in social media are creating new connections, both with neighbors, and like minded peers across the web. So that community is taking place, and connections are being driven.