At the top of the page, it always has this message:
Hello RandomRhino,
Our records show that you have never posted to our site before! Why not make your first post today and be part of our community! Ask a question, give your opinion, or just say hello in the Cafe!
I’ve definitely posted before. Even after I posted in a thread about a few minutes ago, it still says 0 posts.
Anyone know what’s wrong with my account?
Well, it finally counted this as one of my posts. So while my post count isn’t accurate (and this thread didn’t show up when I searched for all threads started by me), I guess it’s one step closer to being kind of normal.
Oh well.
The post count doesn’t include posts from Parent Cafe, CC Cafe or High School Life. Those forums are seen primarily as places for casual discussion, not serious, college-related questions or contributions (though one could argue why posts in this particular forum here count). Anyway, now you know the mystery 
This is a bit off topic, but does anybody know why I need to have 15 posts before I am able to private message someone? Just curious about the rationale.
Spammers were registering multiple times and sending ads via PMs. The 15-post requirement makes it harder for them to do that.