Explain to them about the risks and business plans of companies like Facebook and Google. Give them your email address, telephone number and website/blog and ask for theirs.
If they are only prepared to keep in touch with you through something like Facebook and Google+, then they're not really interested in keeping in touch. Rather go find people with which you are actually both mutually interested and stop chasing after 'obligations'. Life is too short.
> then they're not really interested in keeping in touch
Yes, or they are simply not interested in stories about the risks and plans of Google and Facebook. Or do not understand when you try to explain it.
A significant part of my family and friends is already on that path: only reachable through sites like facebook or gmail. Taking some "political stance" in not joining (how they see it) will very readily be translated in "you are not joining. you are not really interested in keeping in touch with me."
So no. It will work out precisely the other way around.
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...)
If they are only prepared to keep in touch with you through something like Facebook and Google+, then they're not really interested in keeping in touch. Rather go find people with which you are actually both mutually interested and stop chasing after 'obligations'. Life is too short.