why would it need to be a tracking cookie?
although I'm not well versed on how cookies work, basically plain ol' regular cookies keep tabs to what users do and where they go from one forum to the next on this site. An ad tracking cookie goes beyond the TF forums no? (hence the term "tracking")
Yes and no. You are taking a third party's assumption that this is a ad tracking cookie, as well as their definition as to what that is and what it does. According to what they say it is an ad tracking cookie.
What is actually being done is that Quantcast is keeping track of who visits this site, what they do, and so on. It is doing this so that Andy and his team can see what is popular, what is dead, what attracts visitors, what doesn't, etc. Quantcast can be used for advertising but I haven't seen that here. It can also be used as a broad spectrum tracker but, again, I have not seen that here.
Believe me, if anything out of the way was pulled on the membership here I would be the first one firing up Andy's phone (the owner). I can be rather ferocious when it comes to Tech-Forums, having went to bat against several of things Larry pulled out of the air. Andy is a straight up guy as I have spoen with him many times about TF and I am confident that he would not do anything involving underhanded means.