Another recent Nielsen study, however, found that social networks have actually helped increase email consumption. "We decided to churn some quick data to test our hypothesis that 'Consumption of social media decreases email use,'" explained Jon Gibs, VP of media analytics for Nielsen. "It actually appears that social media use makes people consume email more, not less, as we had originally assumed."

To further complicate the situation, tech market research firm the Radicati Group released a report in April which estimated that social networks will grow at a remarkable pace in the next few years--but it also showed that worldwide email usership would balloon as well. "The number of worldwide email accounts is projected to increase from over 2.9 billion in 2010, to over 3.8 billion by 2014," the report said. "However, Social Networking currently represents the fastest growing communication technology among both consumers and business users, with over 2.1 billion accounts in 2010 which are projected to grow to over 3.6 billion accounts by 2014."

The Radicati Group's report also showed how daily email use has been dropping for both consumers and business people--clearly an effect of social networks.

Average Number of Consumer Emails Sent/Received per User/Day:

So is Sandberg right? Are social networks and SMS replacing traditional messenging services? Is it even fair to pit these services against each other?

After all, what is "email" anyway? Today, services such as Gmail now include elements of chat, status updates, document editing functionality, and more--it's impossible to clearly define what makes a social network, and what makes an email service.