I'm the CEO of MangoMap which is a online GIS data management and web map publication platform, we're big QGIS fans and make extensive use of it. We would really love to integrate MangoMap more tightly with QGIS by making the transfer of data and edits completely seamless between the desktop projects in QGIS and MangoMap data/maps in the cloud. We already allow QGIS to get layers from MangoMap using WMS but want to make it much easier to send data and edits from QGIS to MangoMap.
Before we get started though I would really like to know if there are any up to date numbers out there on how many active QGIS users there are at this time. I've seen an article on Spatial Galaxy from 2011 that estimated 100k, but it's my feeling that QGIS has matured and gained a lot of traction since then so the figure is unlikely still valid.