To me its about sustainable growth of membership. Members of the community may actually be in passing: new parents, babysitting co-ops, and sports leagues come to mind.  

Taking your examples above:

Blogs are most often themselves not communities.  If the primary author were to go way, then the group of people investing their time will likely disappear as well.  Related to this, The Apache Software Foundation considers the importance of one or a few committers to an open source software project to determine the health of a community. They call this the "bus" or "pony rule" - what if a bus were to take the few important members away: https://twitter.com/GregChase/status/622885703408443396

Github contributors, if a large enough mass of committers will sustain, or even direct a project even if the governing authority were to disappear.  A great example of this is the Node.js community forking away from the community sponsors because they were unhappy with direction. See the following for latest status on this community drama: http://thenewstack.io/joyent-tries-to-divert-node-js-crisis-with-new-advisory-board/

Power users - if a group of users can, in and of themselves, create a sustainable community or organization without help or organization from the parent company of the product, they could be considered a community.

Everything above is critical mass - critical mass of time, energy, community-own commons content, and enough people to sustainably recruit others.

And, all communities die at some point too, just like stars that ignite when they coalesce enough to spark critical mass, and burn out when there is not sufficient energy to keep them sustained.