Lost and found type stuff is really only relevant to specific trails, anyway, rather than just a general audience.
Different audiences tend to follow each medium. Many people who check the forums do not follow the Facebook groups for the relevant trails, and many people who follow those Facebook groups do not post on the forums. Some check both, but it's relatively few. I think forum traffic is down somewhat due to the popularity of Facebook for discussions, anyway.
I follow a few Facebook groups centered on specific trails, but I don't check every single one for every single trail in the state, and I doubt more than a couple do that.
Benefit of the forum is that the topics that get posted have more permanence than Facebook posts do. It's easier to dig into post archives here than there.