Community-wise, I mostly stick to ADISC. I drop by DD every now and then, though. I'm also on FetLife, but mostly to keep an eye on munch prospects in my area. ABDL discussion on FetLife doesn't quite rise to the level of "community" anyway (IMO).
I first became aware of other ABDLs through the usenet "alt.sex.fetish.diapers" group back in the early 90's. But it was already being spammed to death by the time I found it, and the discussions were oversexed and off-putting to me. I discovered DPF shortly thereafter, but it didn't appeal to me either. At the time, I was relieved to know I wasn't alone, but not particularly interested in talking diapers with other adults.