LinkedIn is the best B2B networking site given the member and quality of membership.
In addition to the 2 good suggestions by Jack and Sophia... Manta has a good forum section called Manta Connect where you can ask and answer questions and network.
"Google +". Just join communities that you need networks from.
You can find gems and potential clients, colleagues, and entrepreneurs in Quora too.
Twitter is not bad, but it will take time to build your network. If you pick twitter, you can just follow all the experts and specialists you are looking for. See for yourself how they roll.
Linkedin would be a good choice for one but personally I can't say I have found it all that useful.
You may find some facebook groups that are of use.
Since you are in ap and web development one you may like is webdesignforums.net.
For general business as well as web design one of my favorites is small-business-forum.net