Social networking websites are used to network obviously. Friends and relatives use it to keep in touch. It is especially beneficial to those that are living far apart. Some of my friends did study abroad and updated their facebook while there allowing everyone to see any pictures they took at the foreign country, or to simply communicate over long distances.
Companies have been using social networking sites to look for potential employees as stated in the article Social Networking Technology Boosts Job Recruiting. What isn't mentioned in the article is that sometimes companies use social networking sites for spying on their employees. Making sure that they aren't posting offending content on the internet since it may lead back to the company. Or they can even snoop on status updates to see if employees are really sick and not just skipping out on work. So on one hand its great that employers can seek out employees now online, but on the other hand employers now possess more knowledge of heir employees and might try to use or abuse that knowledge to their advantage.
Of course its usage will change in time. Presently its a great tool, and only occasionally you hear of it being abused such as cases of cyber bullying or cyber stalking. The BBC documentary "The Virtual Revolution" Episode 4 touches on the potential danger of social networking websites. Some people spend an excessive amount of time on facebook chatting with their online friends or playing applications like mafia wars and farmville instead of spending time their real world friends in the real world. In the future, more children will be exposed to such websites and other virtual world technologies that just sucks them in. Of course it is a worst case scenario, but the concern is valid. What will be the future of social networking and its impacts on humanity? Only time will tell. Perhaps we can become aware of these potential problems and prevent them from ever occurring.