Email was once seen as the scourge of snail mail, destined to wipe the post office off the face of the earth. Now, it looks like email is facing its own competition in the form of text messages, cell phones and social networks.

According to a report from JupiterResearch, permission-based email marketing is starting to lose its impact. In 2008, 44 percent of email users made an online purchase as a result of promotional emails, while 41 percent made an offline purchase. Those numbers are both down from 2007, when 51 percent made an online purchase and 47 percent went offline.

When users unsubscribe from an email list, it's usually due to lack of relevance -- half of email users said they unsubscribe when the offers do not interest them.

The average number of overall emails that users receive is now down to 24, almost half of the 41 email messages the average user received in 2006.

There is quite the generational gap in email usage as well. Younger consumers 18 to 24 years old who rely on text messages and social networks receive only 12 emails a day, while 45- to 54-year-olds get an average of 28 per day, which is the greatest volume by age group.

While overall email deliveries were down, JupiterResearch found an increase in the number of permission-based emails sent. This inconsistency was attributed to more secondary email accounts and marketers sending emails to dormant accounts.