Online networking sites help make networking more accessible. It makes it far easier to find people that are interested in the same subjects, events, or have a common bond (work, school, location, etc.). And the technology/algorithms make suggestions/connections that your brain might not be capable of. Facebook is now going so far to let people know that two people are now friends because they attended the same event.
I think in person networking is very valuable, but online networking gives you that initial introduction to build upon. From there the tie is open to be as strong as both parties want to make it. Plus the novelty of it makes some people accessible that probably wouldn’t respond to a random e-mail.