Try Reddit. It's a different experience, but you can post all sorts of things anonymously and you'll get tons of replies. It's just not a traditonal forum, so idk if you want a forum or not, but I use Reddit.

Sometimes I've gotten 30 replies on a single thread within 2 hours. It's super active, with millions and millions of people online all at once. Let me know if you're interested and I can help point you to good "subreddits" (aka subforums). Some subreddits have 300+ people online looking at them at any given time. I've seen some with 800 people online. You're seriously bound to get tons of replies.