There should be a 'get' or something if they do the dare- Like OP (or LP, whichever), promises to do something if the person does the dare.

Something like: I'll give 5$ to a homeless guy if you do this dare:, or I'll write a salacious comment on your wall talking up your awesome sexual prowess if you do this dare:, or I'll post a naughty pic, or whatever... you get the idea.

Otherwise there's no incentive...

-Rx