http://www.npr.org/sections/thesalt/2013/02/04/171051906/can...
http://eatocracy.cnn.com/2012/08/08/where-does-your-grocery-...
But it's not my argument that small family farms are likely to supply a significant portion of all the produce the median American buys at a grocery store.
Rather, my argument is that there are steady, high-volume buyers for small family farmers. Whole Foods can constitute a tiny fraction of the produce sold in the US (though: they don't; they're extremely popular) and still be one of several means by which it is viable for small farmers to be in business.
And, I guess my point is, this seems to be the case.
When you read this Salon article, you didn't get a bit of a "Brooklyn" vibe from this particular "mid-20s, early-30s" farmer outraged that their watermelons and lavender flowers buttered by the Sacremento sunlight on their less-than-one-city-block farm plot couldn't pay all their bills?
A bigger possible reason farms tend not to join co-ops is that they integrate vertically and do direct marketing both to chains like WF and to farmers markets and, particularly, restaurants. For vegetables, there are a lot of farms like this in the Midwest.
Obviously, if you're growing soy or corn or wheat, it's a different story. But I think nobody is particularly surprised that you can't readily make a living selling crops that are standardized and bid on in commodities exchanges.