http://www.federalreserve.gov/pubs/oss/oss2/98/bull0100.pdf.
Although this is the result of the 1998 survey, the overall composition of financial holdings hasn't changed all that much, as I recall from Maki and Palumbo's paper on the wealth effect. Roughly 49% of all families had direct or indirect shareholdings in 1998; the top 20% owned a little more than 80% of all publicly traded stock.
Christian