Yes, that would be the difficulty in addition to just setting up the system and its rules. What is the likelihood of meeting a FT member on any flight, not even to say a member who is 2P or higher on United and has e500s to use for this purpose?

I guess it's a social networking problem within an already restricted group -- the probability of success gets very low. Just having more members doesn't increase the usefulness in itself -- they also have to take the action of flying on the same flights! Although, if you factor in the main benefit/targeted group of this idea -- family/friends (not FTers) who are flying, the pool could expand greatly. But it would still have to be done through the FT member.