Following some discussion with friends, I made a chart comparing the general prevalence of MBTI personality types with their prevalence on Facebook (using data from this report). The first of each pair of bars is general prevalence, and the second is prevalence on Facebook.
It can be seen that extroverted types are more likely to be on Facebook than introverted types. However, the IN-J types swim against the tide. The chart below provides a bit of a summary.
The third chart shows the results for Twitter. Here extroverts are also over-represented, especially the EN-P and ESTJ types, but not the other ES– types. Among the introverts, the ISTJ type swims against the type, and is in fact the most common personality type on Twitter.