Comangliait's some factor of +frags, and number of posts you have. I know that for sure.
Other possible factors that I can only speculate about
Ratio of posts with significant +frags vs no frags or -frags
Number of posts with XX amount of + frags ex 50 or higher plus frags posts
Threads might give a bigger bonus than posts for all I know.
Pretty sure the 2 threads I have about my config account for close to 2 of my nerdstars.
NakadaI think I read somewhere you need 500 posts for 1 full star
Not quite true
http://www.teamfortress.tv/user/hamaham 17 posts nearly 1 star
http://www.teamfortress.tv/user/Dane 106 posts nearly 2 star
http://www.teamfortress.tv/user/Parapsych 48 posts nearly 2 star
Hamaham got that with that one thread about his father being the heavy. He got at least 400 upfrags from the different posts on that account, to my knowledge.