Do you think iTunes Streaming would completely negate a reason for any of the other services if they had easy sharing features within it?
Not particularly. Features and catalog will play a big role. If I could combine my catalog + a streaming catalog that had all the stuff I wanted on it ... I'd be in heaven. If it's well designed, even better. Hahah. I think some of the other services would be able to differentiate themselves -- but Apple's hardware being so prevalent would give them a big upside and headstart. However, Apple doesn't take things into the web often ... being stuck inside of iTunes kinda sucks in some situations.
I think the key here is what Facebook is apparently trying to do ... make it so that it doesn't matter what service you're on. So if you're a member of service X and you "share" a song on Facebook ... and your friends are members of service Y -- they'll still be able to play the song. THAT's what's really cool. They become the defacto sharing tool and allow more music to be discovered by friends.