I am already on LinkedIn, but what other communities exist online besides this site?
What most people think of as networking is utterly useless. The chances that you will meet someone that can actually do something for you is minuscule. Instead, consider being authoritative and publishing content of your own that is useful to the people you're trying to attract (particularly as customers). Demonstrate how you think, educate potential customers on what they need to know about your industry to make a smart purchase decision (especially if purchasing from you happens better from more educated consumers). Forget simply participating in a neighborhood of like-minded people if what you want from networking is to grow your business. Target your efforts or you're just throwing spaghetti at the wall to see what sticks.
There are various networking Facebook groups which are moderated properly. Quora.com is a great site for networking. Startups.stackexchange.com is also a good resource. Nothing beats Facebook groups though.
Paul has a good point in regards to pushing content. I also recommend to reach out to others when you see a need that you can help with, or want to understand to see if you know someone who can help them.
Example: I recently met Shikha Jain of TechFolds. They make animation, videos, and the like. She was GREAT and the product is solid. She had some sales operational/workflow questions regarding the scaling of sales, which is what I do. I provided some input gratis, and left the door open. In the meantime I have contacts who may need to have video content created, and she'll be someone I recommend.
In short, it's karma. When I meet good people with a good product, I help them however I can, and it will come full circle because I am a genuine value. Your thing may be good people with a broken product, or an idea that needs technical development. Whatever it is, I recommend making sure what what you most like to do is what you're good at, and then going forth with that message to genuinely help people. If you have no practical experience, you'll likely have to give up pay on the front side. Consider it buying your experience. Hope this helps.
Besides Linkedin, I network a lot on Instagram and Twitter, very little on Facebook.
The best way is to sell yourself to your audience with the knowledge you have and you will attract the right kind of people to form your network. For me it doesn't really depend on the resource you use rather what you use those resources for. Facebook, LinkedIn, google+ tweeter.. Etc all are good