The chart below is from Technology Review India. Note that Groupon shouldn’t really be compared with the rest because its revenue per user includes the value of the products and services shipped.
Footnotes: 1.) The figure for Google users refers to the number of unique monthly search users, which doesn’t reflect all the people that see its ads and use its services. 2.) The figure for Groupon users refers to reported "cumulative customers" in 2010. 3.) The figure for active Zynga users refers to "monthly unique users" from October through December 2010. 4.) The figure for active Twitter users refers to a recent report from Business Insider that found that only 21 million Twitter users follow 32 or more accounts. Twitter considers an "active" user to be someone who is following 30 accounts, with a third of those accounts following back. 5.) Revenue figures for Facebook and Twitter are based on estimates from eMarketer, a research firm. 6.) Revenue figures for Zynga and Groupon come from their IPO filings with the Securities and Exchange Commission.