jjsimp: There's always going to bars. However, those of us in our forties or above are referred to as the creepy old guy.
There are bars for people over 40...
Piranjade: Table Top Gaming!
(Blood Bowl, Warhammer...)
I also find pen&paper RPGs to be very social.
That is a good suggestion. Tabletop games have helped me make plenty of friends. It does not have to be something quite as expensive as warhammer or as involved as RPGs though, basic boardgames also work (though if you want to game with modern boardgamers, forget everything you know about Monopoly, Risk & their ilk)
Also, have you considered dancing? There are probably classes for plenty of different dance types (ranging from the very formal to the very informal) in almost every city worth its name.