I have been bugged by friends and family members with their impersonal invitations to join those virtual communities.
I don't really understand it. They have my email, my phone, my address. And we do communicate often enough. What am I missing?
If you feel that you're communicating enough then you're not missing anything. They're inviting you to be polite, but they're not going to be offended if you don't respond to their invitations.
But... if you're wondering why you no longer hear from your kids via e-mail, phone, or snail mail, it's because they feel that they're adequately informing the world of their status via Facebook & LinkedIn. Spouse and I enjoy reading our kid's posts, and we most especially enjoy the knowledge that we don't have to get involved unless she contacts us with a specific question/request.
I didn't bother with Facebook until I had a reason (marketing the book). When I'm done with that (in what, about five years or so?) then I'll probably stop using Facebook and resume wasting the vast majority of my time here.
And what is the "diplomatic" way to tell them to stop sending those?
I would suggest that there is not such a way. It's easiest to leave the lines of communication open and to just delete the things you don't care to read.
Otherwise if you ask them to stop sending invitations, urban legends, joke-of-the-day, or other spam then you may never hear from them again in regard to the things you actually care about.
If you
really want to impress your kids with your hipness, have them walk you through the Facebook process. Pick some innocuous nom de plume for yourself so you can remain incognito. Once you've friended each other then you can read their posts to their wall. After a while they'll forget you're reading their wall, and they'll let it all hang out again like they did before they were teenagers when you could hear them hanging out with their friends in the next room...