It is hard to know what is happening with user numbers in Second Life. From my empirical experience I would say they are growing because I meet so many new people now, but it is hard to find real numbers to back up that opinion. What I do think is coming clear is the nature of OpenSim and the reasons for the popularity of Second Life.
Via blog.nalates.net
The Metaverse Journal
Virtual Worlds Since 2006
I think the only reason you are meeting more people in SecondLife is because there are now people online. Before, there was almost nobody, yet a lot of people were registered. You can’t really meet people in Minecraft, given that a lot of people play it in Single Player, I prefer Mutliplayer myself.
Have not actually used Minecraft (though I’ve seen it on GoodGame) so I can’t comment on that, but with Second Life normally what happens is the latest controversy or major media coverage draws new users. I can remember a huge spike in 2009 after one couple divorced after a bit of virtual infidelity in SL and it hit the news. But how to determine numbers? Many folk have alternate accounts and others just stop using SL without deleting their account.