There are a zillion people and companies that do this. I get a dozen spam offers a week from SEO and SM optimization companies. There is also software that helps you do manage your own SM presence, like Hootsuite. You can use hootsuite to plan and schedule tweets and the like, which can keep your presence active online and allow you to do other things.
I expect at some point in the future, there won't be any actual people on SM (especially Twitter). It'll all be bots releasing content at one another. :)
