There are a bunch of interesting email trends and statistics on the EmailStatCenter. Here is a short sampling of interesting stats that pertain to viral marketing and content sharing in general:

• 44 percent of the largest retailers offer send-to-a-friend functionality in their emails on a regular basis. - Email Experience Council (Nov 2006)
• 88 percent of respondents in a survey reported forwarding jokes or cartoons making it the first most popular category of emailed content. - Sharpe Partners (Jan 2006)
• 56 percent of the same group attest to forwarding news articles (second most popular category). - Sharpe Partners (Jan 2006)
• 89 percent of respondents said they actively share content with others via email. - Sharpe Partners (Jan 2006)
• 25 percent sharing daily or almost daily. - Sharpe Partners (Jan 2006)
• 64 percent of the female respondents said they shared content at least once a week, versus 58 percent of the males. - Sharpe Partners (Jan 2006)
• 100 percent of the end-users surveyed use email. - Datamonitor/Dimension Data (Aug 2007)
• More than 70 percent of the end-users surveyed say email impacts positively on their productivity. - Datamonitor/Dimension Data (Aug 2007)
• 15 percent of Americans describe themselves as "addicted to email". - AOL (2007)
• 59 percent of people emailing from portable devices are checking email in bed while in their pajamas; 53% in the bathroom; 37% are checking email while they drive; and 12% admit to checking email in church. - AOL (2007)