I experimented using only Google+ for a week. I stayed off Twitter the entire time. I didn't even read it. I wanted to see if Google+ could replace Twitter for me. What I found was that too many people have not really migrated to G+. They may not ever.
It's not Twitter. The posting people put up are long form and encourage long comments. In a way it's almost like Google took Blogger and then added circles, but then removed all the formatting. So there, I said it, G+ is just a weak Blogger with some social functions added.
Google has had most of these features in it's various products for a long time. It just took Circles to bring them together. I don't think that's a bad thing and if you are a long time Google user this brings the tools together cohesively. What about for everyone else?
Microsoft has a lot of these same features in it's Live environment but you don't hear about it much do you? Does what G+ do really all that much more compelling that it's going to draw large numbers of Facebook and Twitter users away? Right now I have to say no.
That doesn't mean that there aren't users who have dived into G+. I would put myself in that camp. It also doesn't mean that G+ is a failure or that we won't see more awesome features added. I'm sure Gmail looked weak compared to other webmail services when it first came out too.
So for now Twitter will still exist for me. It doesn't try to do a million things. What it does it does well. What it doesn't do G+ will pick-up nicely.