If you are thinking to use facebook as promoting your website then its good place as huge people are using it. You can get traffic from facebook easily and getting conversions is also possible.
Definitely! It's worth it, but be ready to allot money for it. Try Facebook PPC too. That's a good way to generate targeted traffic.
Facebook applications can be great! It takes a lot of work, though, and lots of planning. You want to develop an application that people will keep coming back to - give them incentive. What is the exact direction of your question? Is it a career choice, something to advance a particular product? It will be easier for us to answer you that way and give you better advice.
yes, facebook is the future - it is most growing social network. Even twitter can not compete with it.