An odd user name...

I was contacted by a user. I received a message and a friend request. Sillyson was his name.
Sillyson had made friends and saved favourites, including one of mine and one of a friend.
I was considering my response when I logged on to find in place the name ##154464856_664c17b4e0ecd and a note that the account had retired.I've seen that sort of number before.
What does it mean?