I'm not entirely against shorter posts (like I mentioned
here), but they should be used fairly rarely/sparingly because there often isn't much reason/point to only posting "Thanks" or "Cool find". They don't really add anything to the discussion and most people tend to skip over them. Sure they show activity, but it's pretty low quality activity. And if you saw a bunch of topics where people were only posting like 2-4 words, would you really want to stick around?
(I know I wouldn't, I would be inclined to think there wasn't actually much discussion going on so I'd go somewhere else. )
So in a lot of cases, I think if a forum has a like/reputation system then it'd be better to use them rather than just posting "thanks" or "cool find". But on forums that don't have those sorts of systems then I suppose it'd be ok once in a while (provided most of the time you are posting much higher quality posts).
Though people on post exchanges? Make a real effort to match the quality of the person you're doing an exchange with. Don't post loads of really short replies on their forum if they're giving you decent length quality replies/topics. Doing that just makes people less likely to do exchanges with you in the future.
And people doing post packages (using either real money or forum points)... People are paying to have posts done. So posting messages like "Cool find" or "thanks for sharing" is rather pathetic. (It's poor quality workmanship and reflects terribly on yourself and the site you're staff on.) On a personal note, seeing posts like that from packagers just makes me not want to get packages from that site since I feel like it'd be a waste (like I'd be paying for spam).