I am compelled to point out that gmail is just an interface to email. Perhaps you meant G+?

Either way, I think email is a pretty open method of communication. As hard as they've tried, FB/G+/Twitter have managed to augment rather than supplant email.

They're also pretty new. I suspect in 30 years, nobody will be using them, but email will still be around. (Probably still using SMTP and battling spam, TBH...)