According to a good and informative 2013 year-end status report by Leadership Network providing statistics on the state of mega-churches in this country, almost 10 percent of Protestant churchgoers attend a mega-church.
According to a Pew Forum report in December 2009, (if there is a more recent report, I am not aware of it), 9 percent of Protestants “attend religious services in homes.”
I actually think that the numbers of people seeing a small gathering in the home as their main spiritual family is much larger (Felicity notes that the stat is four years old). Even many mega-churches today are working diligently to make sure that missional commuities in homes are people's primary source of spiritual relationships.
The public platform of the mega-church is viewed as a much bigger stage because of obvious marketing reasons, but I do suspect that the organic church movement is having more influence than is noted by some.
Scott McKnight has interesting observations about the marketing platform of the mega-church pastor on his blog that closely relates to some of my own thoughts that same week.