Every free hosting site or social web site will have terms of usage that effectively let you waive all rights to the material posted.
For one, they must limit their liabilities, as they are poorly capitalized, relatively to the number of accounts - even if some court would reward each user a mere $10 damages over some data leak, there would be no more Facebook.
For the other, investors love being told "the users can't leave even if they want to" and "even if we fail, we have billions worth of data as your exit option", even if both are blatant lies.
In real life, social web sites can do pretty much nothing outrageous with their user data - they even have to tread careful about placing too many or the wrong ads, or they risk losing most or all users.
Most Facebook users would rather part with all their data than with $100, or they would not be using a free service, so they are very much inclined to run away. So any potential threat of abusing their data does not work out - do something even mildly disagreeable, and you are next year's Myspace. The fate of the growing number of failed social networks has shown that the value of a ex-user base is pretty much nil even where the TOS would theoretically permit a buyer to abuse it. As data abuse only works by bulk (nobody would sift out the 0.001% of commercially exploitable images among all the cats and awkward family shots) you are pretty much safe there, even as a professional.
The risk of using a free service for a professional photographer really boils down to the lack of control over design and context - if the site operator determines it is time for a relaunch, you might over night end up with something quite horribly ugly and dysfunctional, so you'll permanently have to track the state of your page, no matter where you are and how much you work. Even worse, if the site software determines that your page is popular enough among a particular target audience, the rating and grouping system inherent in social webs will automagically place ads and references to apparently related stuff on your page, and inversely, place references to you and your pictures on other pages. And not everything related is related in ways you want to be associated with - in the worst case, it could be a direct competitor advertising on your page, or your pictures and links turning up as eye candy in some context your customers would consider outright offensive.
