We all have fine hair! Interesting. I know a lot of fine-haired curlies cowash w/ no problems, but I could see how our follicles would be much smaller than that of medium or coarse haired curlies and therefore the hair in overconditioned follicles would be sliding out much more easily instead of being strongly rooted in the scalp.
It's a good idea to give your scalp a break from conditioner for a little while and see if the thinning stabilizes. And measure and log your ponytail size!