You raise some good points about Facebook and Twitter going through the same cycle. I can't think of a service that people adopted immediatelyl; strangely, I can think of a few devices.

it's just a matter of ... get[ting] everyone to hear about it.

That's very true as well, but Google+ has something Facebook (and Twitter et al) didn't -- google.com integration and exposure [1].

[1] http://www.allfacebook.com/the-one-google-plus-feature-faceb...