Social networks aren't just useful for procrastinating, they have revolutionized information exchange and social interactions in a number of ways:
Advertising:
These sites have become a powerful vessel for companies and individuals to advertise in a very targeted way to specific markets.
The information that social networks hold about you can be used very effectively to advertise specific goods and services just to you.
The ads you see while browsing a social network will be very closely aligned to your interests and age range.
Similarly individuals can instantly alert friends to upcoming events, or communicate anything as small as a change in mood to a change in address.
Business networking:
"Networking" in business has long been understood as an important facet of staying ahead of current events and seeking out new opportunities.
Vendors, venues, previous employees, and customers can all act as sources of information that can give you an edge over your competitors.
With the power of online social networking, these are accessible instantly, with some access to each of those contacts' individual web of friends, colleagues, and customers.
Businesses, charities, and more recently political parties have all wielded the power of social networks to communicate effectively with their customers.
For the first time in history, these communications can be truly two way, with millions of people in the discussion day and night.