Google+ failed because of they didn't give anyone a good reason to move away from Facebook where such networks already existed, and they insisted on lack of anonymity. The intelligent commentators are all on Twitter anyway.
But the demographic of teenagers, bored housewives and creepy middle-aged men on the internet is very large, and they are on the side 24/7. Pretty much everyone else who uses the internet uses it for work and hardly uses Facebook. But they trudge along to Facebook a bunch of other people who of course need to stay in the communication loop.
Email is not going away because no other system is a wide-spread. Slack could come close, if only they took away their silly way of logging in. It was a shame that Gmail is rooted in email. Heck, if Gmail changed their backend to something like Dlack, 50% of internet users would be no longer using email and they wouldn't even know it. I dream of a Slack to Email gateway.