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.
The reason I started Apache Leads way back in 2003 was that I was a Diamond level distributor for a San Diego based MLM company and had quite a large business. None of my associates had any access to leads, so that's how we got started. The demand for leads grew so fast that I had to focus on getting quality, affordable leads in ever increasing numbers.