Considering the health issues caused by loneliness, it makes sense to me that doctors would want to do something about it, and what can they do but provide medical solutions?
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.
Watch this as many times as you need to.
In the nineteenth century, teenagers were trekking across the continent with young wives and children in tow. Look at where we have arrived: mewling man-children.
Go join the Marines.
Imagine if workers had actual careers, that didn't bounce them from city to city (unless they wanted to relocate).
How about if housing had minimum codes that encouraged actual privacy. (Such as being able to take a shower or do the dishes at 2AM without anyone around you hearing anything; also to doing them at 10PM without waking morning people and at 8AM without waking night people.)
Also if 'rent control' / housing cost control meant that if the market were over a given price more housing would be built in that market to drive the price down properly.
Housing also should not be an investment, that encourages poor behavior in all sorts of ways.