The percentage of U.S. millionaires using Facebook in 2011 has nearly doubled to 46%, from 26% using the social network in 2010, according to "Social Media and Affluent Households," a Spectrem eZine report released Thursday by Chicago-based research firm The Spectrem Group. Millionaires are defined as having a net worth of $1 million to $5 million, not including primary residence.

Similarly, 47% of ultra-high-net-worth investors, those with a net worth of $5 million to $25 million (again, without primary residence), and 55% of the mass affluent, who have a net worth of $100,000 to $1 million, are also using Facebook.

This is one way that financial advisors can reach this plum demographic. Blogs rank as another good way. Nearly one-third (30%) of ultra-high-net-worth investors say they either read or would read blogs by trusted financial advisors. The percentage is 20% for millionaires and 21% for the mass affluent.

In addition to Facebook, LinkedIn is also popular among wealthy investors, with 19% of millionaires, 26% of ultra-high-net-worth investors and 22% of the mass affluent using the service. Twitter usage was smaller, with 3% of millionaires, 6% of ultra-high-net-worth investors and 5% of the mass affluent using the service.

Spectrem eZine reports are 20-40 page multimedia reports that combine written text, analysis and video based on Spectrem's research with wealthy households.

-Jim McConville