I recently tried signing up with Linkedin but had issues with regarding to registering as a company instead of myself. They seem to have some rules against not using your actual name. If anyone is using it as a company I'd love to hear from you.
Although I don't put a crazy amount of time into social media I've always found that Facebook is more powerful for converting into sales. I treat it very much like I would an email list, it's another way to reach my existing customer's and keep them up to date with new products/services.