I don't really use any of these that much, but from what I can tell, the only real difference between Facebook and Google Plus is that G+ doesn't have relationship status updates. Besides that, they seem to pretty much have the same function like the Like button on Facebook and the +1 button on Google serves pretty much the same purpose. Also, I think Google Plus has a more serious tone to it, with regards to the general atmosphere of it and the content that gets passed around, am I just imagining this?