I'm sure there has but I can imagine just how awkward it would be. It would be like a retarded school trip. But if you mean just two people then ofc. Lots of people here have met each other. A few people here recognised me in real life and asked me for my autograph. Dreams really do come true when you just BELIEVE.
Lots occur I imagine. If you get along online, then I don't see anything wrong in giving it a chance and meeting up. Fun day-out, good conversation and getting to know new people (potentially improving communication skills).