Back in July the Sierra Club launched their "social network for outdoor enthusiasts." I wanted to give it some time to get going (it is still in BETA), and just a few months later, I'd say the site is off to a good start.
Hikers can post trail descriptions and updates can be added by subsequent visitors. Links to maps and websites as well as Google maps-based trailhead directions and trail photos are often included. Here in the bay area we have a plethora of on-line hiking information for our immediate area and beyond, so I don't know how useful bay area hikers will find the trail descriptions. Perhaps the best attributes of the site are the ability to comment on trail conditions and find information about out-of-state locations where on-line information is lacking (do places like this still exist?).
Anyway, check it out at http://trails.sierraclub.org/index.html