The hashtag support was added to Facebook recently as part of a greater push to keep track of trending topics across the social network. The OpenTable integration is part of a broader partnership between the two companies that allows Facebook users to make restaurant reservations right from the Facebook app, using the 'Nearby Places' function as well as restaurant fan pages.
Facebook says its integration is far more seamless than with OpenTable's other partners, like Yelp and Google Maps, which require you to open a browser window to complete a booking. All reservations can be made without leaving the Facebook app. In fact, you don't even need to register with OpenTable to make a reservation.
Reservations will not generate posts on Facebook and reservations will not be sharable with friends.