80% of Americans earning more than $250,000 pa are social media users

"The reasons for going online are, as most other industries already know, compelling. Eighty percent of people with an income of over $250,000 are social media users according to Unity Marketing research, and 50 percent have used social media to learn more about a brand or see new products. Data from small business consulting firm Lex Consulting shows that those earning more than $150,000 are the only people spending more than they did before the recession. And a 2011 Digitas study notes over the next decade digitally entrenched millennials will become the next major luxury buyers—and should therefore be targeted now."
Source:  AdWeek, 12th September 2011
Note - While they don't explicitly say it, I'm assuming that the research quoted is for the US only