My best advice would be if you'd like the forums more active, try to post more, like others have suggested. There is still a trickle of new members. Maybe something will grab someone's attention and make them want to stay a bit longer.
Even if no one else joins, there are still the few active members left. Have fun with each other.
I know people come and go and old members check in once in a while. If you were having fun with the people who still visit, they might be persuaded to join in, too.
I agree with PPT looking into other areas besides Neopets. I've suggested this in the past, too, as well as more activities that weren't Neopets-based, because I was having a lot of trouble going to Neopets on the computer I used to use. I'm only a forum admin, though, so I can only suggest things. I've tried to help things become more active in the past and even though a lot of people had fun, there were complaints as well. I guess you can't please everyone? You shouldn't rely on the forum staff for activities, though. Back when I first joined PPT, it was the regular members who came up with a lot of fun things to do. There were a lot of giveaways and games and such that they created and ran on their own. You want to play Werewolf? Start up a game. You want to role-play? Create one. You want to play some other game or do some other activity? Post it up and see what happens. Keep in mind the number of people active at the moment and tailor your activities to that. I wouldn't suggest spending a lot of time coming up with something crazy indepth. Start small and build up. If it doesn't work, try a new idea. Even if no one replies, you can say you tried.
It's everyone, the members and the staff, who make up PPT. You can't go back to the past, but you can make do with what you have.
I created a forum for my daughters to use to help them learn about posting and interacting online until they are COPPA age and for myself to practice and learn phpbb. It's basically only them, my husband, several friends, and myself. The girls are pleased as punch, though. They post to word games, write little stories, draw pictures and post them, PM people, and play arcade games trying to beat each other's high scores. They could care less that there are only a few people posting up. They are having fun with each other.
Every once in a while someone will come across our little forum and sign up. Sometimes they post up once or twice and disappear. Sometimes they join in the fun and hang around. Either way, we're glad they stopped by.
It's all what you make of it.