Social Media Competition

Cruise Critic is a web site all about what the name implies.  One of the key features of the site is a message board.  Think Facebook except all about cruises.  The message board is divided into sections for each cruise line and each ship of that cruise line.  After a couple of clicks you can find people who are sailing on the same ship and intinerary as you.  It's called a "roll call" in Cruise Critic parlance.  Jan uses it to find partners who might want to join us on private tours.  Princess decided to start their own roll call within their Facebook page.  I'm not sure what the advantage is to them. I suppose it's easier to pitch products to your own customers inside that environment, but I wonder if people who use Cruise Critic, which is independent from any cruise company, will be quick to leave that bulletin board in favor of one that is owned by a cruise line?