What is in your terms of service for your website?
I’m no lawyer but you are strongly advised to consider a Creative Commons licence - which essentially involves users giving up exclusive rights to content when they post on your website, e.g.:
3. User Content License
User contributions are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
“Creative Commons licenses are legal tools that help you grant copyright permissions to the general public.”
So I read that as posters losing exclusive copyright and it becomes material which can be used elsewhere so long as the website is attributed.
(I’ve just noticed that licence is slightly out of date, btw, there’s a new version 4.0)