It may not be as popular to you but then again, correct me if I'm wrong but have you been on this forum from that period of time till now to be able to monitor it's popularity rate? It's good that you said your observations might be a bit wrong which it is because although this place doesn't announce the number of participants that flocks into this place, this place boasts of many users.
There are other forums that are also in circulation but I don't think they are as popular as BitcoinTalk.org.
- Bitcoin.com
- Overlockers.
-Beermoneyforum.
-Cryptocurrencytalk.
-Bitcoingarden.
-Altcointalks.org
- Coinforum.de.

 All those forums listed above all discuss cryptocurrencies and Bitcoin in some way yet the only one I've heard about is Altcointalks. In terms of forum popularity, BitcoinTalk.org takes the lead.