Here is the short answer: yes. They definitely can!
Many employers and even the self-employed (like myself) see social networking sites as a huge time-suck that reduces productivity by distracting workers during their day and causing them to spend hours ‘liking’ posts and watching videos of cats playing the piano. But social networking sites could actually work for you professionally if you’re willing to make a few subtle changes.

For the self-employed, social networking sites can make or break their success.
First of all, if you plan to make professional connections through Facebook, Twitter or Google+, it’s time to clean up your act. I always cringe when people use their professional Facebook account to go on politically incorrect rants or post sexist joke after sexist joke. It simply isn’t the forum; if you want to make a separate, personal Facebook with fewer (and only personal) friends where you can let that side of you loose, post photos of yourself drunkenly making out with an unamused police officer or mooning the camera, then by all means, do so… But do not do it on your professional account. That’s not to say you can’t let personal life bleed through; just don’t do things most people would consider unsavoury and/or unacceptable in a professional setting as this is simply inappropriate. You’ve turned your account into a professional setting, so act like it!
Advertising on social networking services is a brilliant idea. Costly? It can be. But it’s definitely worth it as these sites are frequently set up to find the right people to whom they should show your adverts. Make a Facebook page for your business, too, and ensure it has its own Twitter feed. Create an online presence for your business, and you’ll be able to attract those in need of your services better. Don’t hesitate to post from your business account and advertise your services a little, too!

Meeting people in person is no longer the only way to forge a professional bond.
Making industry contacts has actually been facilitated by Facebook and the like. I am working on a science-fiction anthology at this very moment, which I wouldn’t be if it weren’t for Facebook. When a man added me on Facebook, sharing friends from the literary world, I accepted – and a couple of months later I embarked on this project with him and a slew of other lucky authors. Don’t be too private about your professional account; the more people see you and are involved with your account, the more your name will get around. And you never know; someone you’re not personally close to may post a link to an opportunity that’s just perfect for you. At the end of the day, every door closed is a door behind which wonderful things may be going on. So open as many doors as you can; if someone turns out to be a nuisance they can always be deleted and blocked later on as needed.
Social networking sites have done a great deal to dilute some parts of human interaction. But using them to network professionally could very well prove to be the lynchpin of your professional success. So get out there and start using them accordingly!