I think it's real nice idea, the theatre i work at has a page, just search for the Wycombe Swan and your there, get update's on performance's and discounts on tickets. I think it really is a good way to get new target audience's into the theatre and gain an interest.
Also i like the show's that advertise on facebook etc, it was lovely to watch the weekly cast blogs for Spring Awakening, keep's people in the know and gains interest. I personally have a group for a show im working on at the moment (Writing at the moment), because i think it would be nice to let people in on the journey a show (if anyone's interested).
Just my two pence worth