It is best to assume that your IP address isn't private information. Every single site you visit will see it because it's the 'address' to which the server's response is sent to.

What is the utility of correlating someone's IP address? General location? Whether users share IPs?

This kind of thing is done by everyone. Social Networks and Ad Networks.

Have a look at this: https://panopticlick.eff.org Your web browser is likely to have a fingerprint making it unique across websites. Sign up to one site with your name, then that can be easily correlated to you.

IP addresses (can) change all the time, so the attacker would also need to have a way to find out who had which address at the time of the post.

As ogdan mentioned, if you're posting something you need to be that concerned about, you just should be using TOR.