It's easier to bootstrap a new network if you can take your data with you. This is how Facebook/LinkedIn got much of their early viral growth. They would scrape your email (at that time the biggest network) for contacts and invite people you knew.
Just a nitpick, but FB did that without any real user consent. This would require that companies not do it at all. Rather that the user himself would have to take his data to the new company and explicitly give the new company consent to host the data. That's the way it should work. The user should always be in charge of her data.
