There's no way to know what most people are using on this site.  Most of the modellers who come here are creating stuff for Poser, so most of them model with software that converts to Poser or exports to Poser easily.  I think there's some modelling software out there, like Hexagon and Carrara, that have a direct link to Poser or Daz software in it, so you could make a model, and then export out directly to that software.I would think most Poser modellers are using one of those apps.  You sometimes see Carrara and Hexagon advertised with Poser or Daz models, and the Daz company owns both of those modelling software.  As the guys above have mentioned, there's people here using everything from Silo to Maya, and C4D to Lightwave, but I don't know how many of them are vendors here.  Almost all the vendors here are strictly modelling Poser stuff, so it really doesn't matter what they use, as long as the models work well in Poser, which isn't really hard to make happen, unless you have to rig the stuff in there (YUK).

The most popular 3D software in the world right now, in terms of licenses sold, might be Zbrush.  I've not been in any studio to date where there wasn't a GoZ pipeline active, regardless of what the primary apps being used were.  With paid software, you can measure popularity in terms of units sold, active licenses, and render nodes. I think Zbrush wins the software licensing battle, and Vray probably wins the renderer battle. But if you are trying to measure how many people use the free stuff, then that's harder to get right.  It's a toss up between Blender and Sketchup.  Blender might have as many or more active units out there as Maya or 3dsmax, but there's no real way to know.  I hate 3dsmax, but back some years ago, there were some figures floating around saying that Autodesk had about 350,000 active 3dsmax licenses, which was by far the most of any app in their software lineup, including Maya.  I don't know if that's still true today, I doubt it.  This was before Blender got really popular, and Zbrush was still only a sculpting app, and retopology had to be done in another software.