This might be a good start:
http://anarchy101.org/2609/is-there-website-or-forums-for-anarchists
In addition I would add that some folks I know engage in a lot of Reddit stuff, but That format is not one I find decipherable, so I don't know quite where to send you. Maybe start at the Libertarian Labyrinth?
An online "community" is something to be explored. Is it really community? There are people on this whom I respect, and whose answers I tend to appreciate. There are some folks on here I have even had the pleasure of meeting in person, and who I look forward to seeing whenever we end up in the same place. I would distinguish this from a community.
If we are taking the term community as a positive, then it ought to encompass an reciprocity that is just not possible via the tubes and clouds. It would encompass an ability to nurture the young and take care of the ageing. This (as an example of online community) is not that - wonderful, yes, but we are geographically dispersed in a manner that precludes community.
If you want a little bracing curative to that, here is something:
http://theanarchistlibrary.org/library/john-zerzan-the-nihilist-s-dictionary#toc5
