Nothing is preventing it, but 3rd party operates on a copy. You are still owner of the data and it is on one place which makes it easier for you to access it, share it, backup it, analyze it. So, this doesn't prevent reselling in general but prevents data locking. From there, I guess its not that hard to demonstrate which 3rd party sold your data and sue them. It also mandates nonproprietary data formats.
All that is much, much better than what we have now.
> From there, I guess its not that hard to demonstrate which 3rd party sold your data and sue them.
But it doesn't? Obviously every site's TOS will say that by providing them with your data they can use it for all sorts of purposes. If you sued, you'd lose.
And you're generally going to want to make your data available to the various services requesting it, because otherwise most people won't see your posts and comments on their preferred platform.
Your site, the source of the data, could also include a TOS. Plenty of working examples in the commercial world where licensees are allowed to use data but not compile or resell it.
So I actually started researching this, and it turns out that, by the principles in Field v. Google Inc. (2006), neither side would have an enforceable TOS, but that a user making their social data available on their host and not attempting to block any particular crawlers with robots.txt would create an implied license that would allow social media sites to store and display the posts. Which is what allows Google to display information, snippets, and images from sites it crawls.
Facebook couldn't enforce a TOS because the hosting user had never gone to facebook.com and created an account, so the user never agreed to a contract. But a user couldn't enforce a TOS either because the crawling was automated, so Facebook wouldn't be agreeing to a contract either. But Facebook would be allowed to use the data because that's what a user is inviting by making it publicly available to crawlers and not doing anything to restrict access to Facebook.