Twitter's growth is increasing. Facebook, with 56%, of the internet population is fairly locked in. But either way there are far more people on than you will ever reach to have to worry about running out.
I've noticed a thinning on new twitter followers. Could it just be me? Or is there reluctance with the date of the 25th/Hanukkah?
Presumably when you first started there was an active population who found you through existing channels which is now gone. How would someone find you today? Why would they follow you? And yes, holidays can have major swings in all traffic.