It can be if those photos get you to make a reservation at one of the restaurants in OpenTable's network.

Chances are you won't knowingly spam your friends to tell them to use OpenTable. Maybe you had a great experience with it and you post it once on Facebook. You're not going to continue posting how amazing your experience with OpenTable was. You MIGHT think some/most/all of the dishes you eat at restaurants are amazing. You MIGHT post those dishes on Facebook. If those dishes are tied to restaurants that OpenTable serves, they just landed on an amazing advertising model.

instagram sold for a billion.

Foodspotting reportedly had 2MM users. Depending on the % active, etc. OT is picking up users at $5/per is probably in decent range of what they typically will pay for an active user through other marketing channels.

Add in the talent, the app, etc. and they get a good deal out of it as well.