The one forum site that still seems to be active is prospers.org, where pensioner was active as late as 2014. Not much conversation about prosper anymore, though (The old timers still there have thoroughly soured on p2p lending).

prosper.com itself is still in business. AFAIK they have pivoted to a much more conservative lending model. People who invested before 2010 or so generally lost money, some of them a lot of it, but the current model may be sustainable.