How many forum software are out there? Unless your forum is way better than any other one out there, which I really doubt, because custom-made software without large deployments tend to have many rough edges that took many years for the major ones to clean up, there is no way the software will evolve without strong leadership. OSS that really works is driven by a strong community. The idea of releasing software you no longer want to support to an OSS community is absurd. Releasing as OSS will take you more effort to start with than if you kept maintaining it by yourself. Developers don't come by themselves. You first need to attract users, which means supporting it for a while. Then some developers will come, but they will need direction. Sure, in a few years, the project might fly on it's own, but not tomorrow or any time soon.
This really seems like a modern cliché for the idea of just placing something on The Internet and making millions. That's the kind of thinking that brought us the dot-com crash.
Seriously, if you don't intend on supporting it, do the world a favor and don't add yet an other forum on sourceforge. Delete the code from your computer and never think of it again.