I honestly wouldn’t mind having a “like” or “thanks” button here, but the one that comes with Discourse (just like on Reddit) seems to influence sorting, which I would not want. All I would want it to do is add a number below a post and to optionally notify the user (though they could turn such notifications off).
As the software currently exists, I’d just stick with thanking people in posts. As a standalone post, I’d probably limit my thanks somewhat, using it for bigger thanks. But including the thanks as part of another post is an option, too. As is thanking someone for several posts at once… eg. “Thank you for your help. And thanks for the other times you’ve helped me, too.”
Maybe I’m odd, but I’d save PM thanks for the most special, or when the thanks gets a bit personal. I know that, if you PM me a compliment/thanks/etc, I’m going to want to reply back.