Web 2.0 doesn't have a clear definition. But some of the things that keep getting mentioned are:
My teenage niece uses myspace. She IM'd me last month asking how to hack past her school's firewall so she could get to myspace. I don't think anyone is hungering to get to Orkut or Yahoo 360 that badly.
My view on Google and Yahoo's entry into user generated content communities is that it has a top down quality to it that are similar to existing popular word of mouth grown services.
These popular sites with millions of users are like Google used to be, disruptive web destinations. Why duplicate when you can buy?