Posting your content on multiple sites is very common. Whether it's your own personal site, facebook, twitter, instagram, patreon, etc. I know web-comic artists that post on most of these as well as one or two web-comic specific sites.
The specifics will depend on the site's terms of service. But in general, if they don't pay you for your content, they can't expect to have exclusivity. So you can post it wherever it will be accepted.
It might also be worth noting that most websites with user-generated content do not want to be liable for that content. So they are very explicit about that it's your content, that you publish it, and that they are just distributing it.
One thing you should keep in mind, though, is that when you publish something online, it may mean that you can not sell exclusive rights to anyone in the future. For example, according to the ToS, when you post on stack-exchange, the content is released under a creative common license. Anyone can then copy and use it (with attribution - CC BY-SA). So if you publish a story here, you may not be able to sell it to a publisher later on.