I think there's more to it, too, than JUST the convenience factor... it can be so insidious.
"Hey, my old boy/girlfriend from college on FB!!"
"How interesting to be able to catch up and reconnect, I'm curious and we always had such great conversations."
"Wow, they're struggling with some of the same things in their marriage that I'm struggling with. We can encourage each other and share tips and vent."
"I can't remember the last time someone really understood me and loved me for what I am the way they do ..."
Slippery slope, y'all. And it didn't even necessarily start with evil intent on either side. Just too easy.
"Show me a completely smooth operation and I'll show you someone who's covering mistakes. Real boats rock." -- Frank Herbert