They are important because they contain a significant portion of many people's address books. When Facebook was offline a few days ago, I had no way of reaching about two thirds of my contacts. And I'm someone who's made a significant effort to move off of Facebook. There were people I wanted to contact that day that the only way to reach them would have been to ask mutual friends for other contact details. And there were a few people that I either don't have mutual friends with or who our mutual friends were also only reachable via Facebook. If legislation aims for some form of "interoperability" the main condition should be that, if Facebook were to disappear again, I would still have the ability to reach all of my Facebook contacts via another network.
No, because we're different people. Some people are drawn to Facebook's firehose feed, and there's not really anything you can do to stop them in a free world. It's a disgusting, albeit perfectly legal exchange of goods and services. Microsoft and Apple fought long and hard to make sure consumer protection laws like that never saw the light of day.