Facebook used to be an easy source to tap into, but since they have formed a public company with shareholders who need to be kept happy, Facebook have changed their Terms of Service several times recently and have clamped down on a number of things that used to make lead generation relatively easy. They have discouraged siphoning off clients to external web sites and CPA offers, and have raised the cost of advertisements that do this. It remains a viable lead source however.
Networking, as it used to be called, is a pretty thankless task. The conventional approach to networking was to start with the network of friends and colleagues that you already have; friends and family, your existing clients, your contacts at the squash club and so on. Most people feel uncomfortable pitching to friends and family, unless you have an unbeatable idea or concept, but if that was the case, then lead generation would not be a problem