In June 2009, I asked a lead OpenSim developer about my estimate of 15,000 active users of Second Life's open source spinoff of Second Life. This source told me that 15,000 sounded about right.
In January 2012, extrapolating from this month's OpenSim growth statistics, HyperGridBusiness' Maria Korolov posted a recent update: "[I]f you take the 9,700 users that we counted, add in the 6,000 or so users that are missing, we'd be at around 15,700 active users -- a pretty decent number." (Emphasis mine.)
And I agree, that's a pretty decent number for a 3D open source platform. It also suggests, as I wrote last week, that OpenSim doesn't seem to be growing. Then again, if it was about 15,000 in 2009 and around that number now, neither is it losing users. (And I'd be wrong in speculating that OpenSim usage was dwindling.) At the same time (as reader James OReilly points out), OpenSim's status on Google Trends has been declining in recent years (as the chart above shows.)
Again, this is not a criticism of OpenSim, which is a very useful platform in many cases for a limited subset of developers and consumers. However, it needs to be pointed out for an important reason:
The future of open platform 3D worlds is still riding on Second Life. If SL fails, there is still no commercially viable platform waiting to take its place. Which again brings up a core irony: OpenSim seems to be undermining SL's main revenue model. So one hopes for their own sake at least, its 15,000 users are at least paying for a Premium SL subscription.