Even the most unsociable characters have topics that will get them involved and talking. Find out what those topics are and then just keep an eye out for any posts where they come up for an instant talking point.

It also helps a lot to talk to players of really social characters and see if you can get some CR going there, because a more social character can drag a less social character into things.

For instance, I once played a really stoic and quiet military type. Any sort of talk about military things would attract his attention because that was his area. Eventually a couple of other characters in the game started a training school and he signed up to be an instructor. He then ended up with a few really friendly students who decided that he needed to have more fun and would go seek him out to get him involved in things. All of this allowed him to get CR and get more involved while remaining IC.

It's definitely possible to have a decent amount of CR! You just need to put more effort into it than you would with a social character.