Facebook, sort of? My now-FIL and I had the same kind of benign brain tumor about a lifetime ago, and we were both members of an online support group for it. In 2004, when I was almost three years post-op, I arranged to visit my now-FIL at his home when I was in England for a Christmas party with some non-tumor friends. I even saw my now-husband's old bedroom (though I still didn't know anything about "the son")! Anyway, my now-FIL and I remained in contact even after we moved on from treatment and resumed our normal lives.
Fast forward to 2011, we'd both made the jump to Facebook, and my now-FIL suggested I add his son as a friend because he thought we'd have a lot to talk about. We'd been 'chatting' with each other in comments on my FIL's posts already, anyway, so I sent a friend request along with a message telling him his dad told me to add him... A year and a half later, he collected me from Heathrow for a long visit. A year and three days after that, we were married.
So, yay for benign brain tumors!