World's of Fun (my local Cedar Fair owned park)started doing a blog on their site this last year. It was fairly popular and their PR has recently added MySpace and YouTube profiles to the mix. It has taken some time, but I think at this point all but Disney have realized the value of online PR. They have a nice website, but they don't create extra content in the same way Universal has HalloweenHorrorNights.com or how Busch had DeathJockey.com. At the very least, I'll say I haven't come across anything like that. I don't think they have realized how to create an online experience for 16-30 year olds yet. Hopefully that will change.
Yes, parks can easily develop their own sites to promote themselves and their products. The smart ones will add visitor forums to give said visitors a "venting spot" for any and all kinds of feedback about the park involved.
The smartest parks will not only LISTEN TO, and ACT on such feedback (well, within reason anyway), but will also pay attention to what's being said about them on third party forums such as TPI.
I have every reason to believe, for example, that Sea World monitors this and other forums where people are discussing their parks. I know this because one of my acquaintances in IMATA made direct reference to something I wrote in one of my posts here. Whether the parks so discussed actually take serious note, and/or act on anything they perceive as a real problem as a result, remains to be seen.
If they're truly smart (and I have no reason, outside of some parts of the Entertainment department, to believe otherwise), they will.