Earlier this year, I decided to tidy up my internet presence by deleting various website accounts I was not using anymore. A primary motivation for doing so was that they were turning up in Google search results, and ranking higher than pages I was actually actively using.
In the case of one site, I spent a considerable amount of time searching the entire site for a way to delete both my account and the things I had posted with it. Finally, after determining they simply hadn't put any such option in place, I contacted their customer service email. The first person to respond simply told me to delete it, apparently unfamiliar with the site they were supporting. The second person said they would have someone look into it, then never contacted me again. Finally, I tracked down the email address of someone in charge of the whole division, and he responded right away and was very helpful, taking my data down as soon as I confirmed I was the author.
Other sites, I never even got that far: they simply didn't respond to my requests for data removal.
If a website wants to hold onto user-submitted data and not allow it to be removed, fine—but, they should point that out very clearly when someone signs up so they can choose to avoid it. But a better site, a site people can join and trust, is one where users control their own data.