KLM has a meet & seat program, where you can see if you have a linkedin or facebook connection in common, with the person sitting next to you. Are other companies utilizing social media in the same way?
AirBaltic has a "Seat Buddy" program where they let people choose to sit with other passengers with similar moods / interests.
Arguably, Airtroductions has tried to get dating in flight happening, but it's dubious as to whether they ever properly got "off the ground".
Planely lets you reach out to other passengers on your flight too (although they also need to be planely members)
Finally, WeMetonaPlane allows you to try and contact that special someone you made a connection with. Again, presumably relies on the other person using the site too...