I guess that's not really coordinated social change, but I think tweaks to the context in which we live can have huge, positive social impacts.
"Redesigning neighborhoods to be walkable." Full stop.
In my opinion this includes /planning/ for Parking to have it's own space, preferably /under/ everything. (You don't have to dig down, it's an option build up to begin with!)
Also, relegate /any/ smoking to only designated shelters with negative draw and scrubbed exhausts. It should be a 1 day's pay fine + 4 days of community service for a violation.
I do think a backlash is already underway against Facebook, at least. This is a step in the right direction, as social media seems to intensity loneliness. The internet can be a tool for community-building -- I see other comments mentioning meetup.com, and I've met many people through Twitter and other online social channels -- but none of the solutions (by my estimation) seem quite there yet in terms of helping people find groups that might suit them and providing a means for keeping people together long-term.
I actually think the most important solutions rest primarily in land use. The "new urbanism" ideas of walkability and mixed-use buildings seem to encourage more casual interactions (which ultimately becomes deeper interaction over time) as people live their lives. On the other hand, I live in Manhattan and I'm still lonely. It's hard to say what will work.