What it could have been: Facebook. Or something really similar.
Still extant? Yes. In fact, it still has something like 100 million members, although very, very few of them are in the US. It did, however, become a big hit in Brazil and India.
What went wrong? Google launched a Facebook-like social network a month before there was a Facebook, and quite a few people liked it–but the company failed to understand that the idea was incredibly powerful.
Random trivia: Orkut is still in beta.