Agreed. The reason I said economic is because that's what the OA seems to feel like he deserves, but to be fair he does leave the door open for more clever incentives. However my thesis would be that the OA is already a lost cause and not someone worth pursuing.
As far as the incentive to write a review on TripAdvisor, from personal experience I'd say the overwhelming incentive is to either benefit or stick it to an establishment you visited.
> What kind of economic incentive could Facebook afford to give you that wouldn't make you feel like a worthless cog in their machine?
Must the incentive be economic? Facebook provides value to its users through a social incentive. Likewise, the incentive to share an article on HN or Reddit is your Karma score and to engage in a lively discussion on the topic. Neither of the two is economic.
An interesting point that you failed to make when discussing the economics of the situation is the following, regardless of whether or not a user contributes to TripAdvisor or Qype he can still peruse the site. So my question is the following, what incentive does a user have to contribute reviews to TripAdvisor or Qype versus simply reading others' reviews?