I think it would be great to have some indication of how many active instances of XWiki are out there in the wild. The idea is not so much to know the figure but to see how this figures evolves and thus to see if we're doing things right. It will also give us information about how quickly or slowly our user base upgrades from version to version.
Here's my proposal:
* When installing XWiki on a clean DB (ie first time install) generate a unique id and store it in the DB (similar to the version we're storing in the DB but using some UUID). Thus when an install is upgraded the same id is preserved.
* When the Extension Manager is used and thus connects to extensions.xwiki.org (default extension repository), this unique ID is sent too.
* In addition the version of XE is sent too
* On extensions.xwiki.org side, we only log the unique ID/XE version **without** logging the IP or any other information thus ensuring that the ID remains completely anonymous
* We display a counter on xwiki.org about the # of active instances of XE with a graph about XE versions used