Another simple explanation of editing trends
@First Light: The main purpose is "to write a world-class encyclopedia", this seems obvious to me. The community is a means to this end, this is something we should never forget - certainly we need a welcoming and friendly community so that contributing to Wikipedia is a positive experience and people aren't driven away, but we must always have in mind the consequences for the real purpose of the project, which is the encyclopedia product. People "who come here hoping for a MySpace/Facebook social culture" need to be told that Wikipedia is not about the community - the community is a great thing, without a doubt, but it is a community with a specific purpose and a goal. Wikipedia is about its content. I think that "editors" who are in fact not editors but mainly talkers who don't really contribute to the content or to its maintenance can be detrimental to the project. Of course we need "meta talk", which is what we're doing here right now, we need to discuss the project and its policies, but people who become focussed on only this and lose the base of plain article work tend to get the project not a millimetre further. So, I agree with Llywrch that attempts to improve the "community spirit" should always be tied to actual article editing - in Wikipedia, it's not just about "how to be nice to each other" but "how to improve Wikipedia whilst being nice to each other" ;-)
As a main reason of dwindling editor activity, however, is not connected to questions of community spirit at all, but to the simple fact that easy, popular topics are now already well covered in the biggest Wikipedias (see also Thread:Talk:March 2011 Update/Lack of easily filled gaps), I think that in the future editors need more help regarding how to write good articles for niche topics, and how to find and use the increasingly specialist information needed for filling the remaining gaps.