In 2012 Facebook offered you the chance to message its users for 65 pence, if they were not already your Facebook-friend. I found this pretty useful and sent messages off to my future housemate (who didn't know me), plus a new cousin in Sydney and one in rural Massachusetts.
The pennies spent guaranteed my Hello would reach their Facebook inbox and not go to a hidden folder.
I had great responses from Sydney and rural MA, and consider it money better-spent than had I dusted down the stamps and posted a big long letter. In the event, the cousin in Sydney never replied to the Facebook message, but tracked me down on my website and sent me an email.
So a good experiment. Update, 2015, it's back and boy have I made use of it.