The main reason why leading world companies create accounts in social networks is to attract traffic to landing pages and websites. YouTube's video hosting by the number of search queries is second only to the Google search engine, and you can attract some of this huge traffic flow to your site / landing page by simply posting your video (and related links to your web resources) on this hosting.
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.
Pay Per Click or PPC can be very effective in the right hands. If you do it the wrong way it can be a fast track to financial ruin. Unlike Facebook Ads where you can see how effective your ads are within minutes, Google AdWords are much less responsive and you can use up a lot of cash before you hit the winning combination. Concentrate on being very focused and specific. If you are not, you will get lots of clicks on an ad which sounds right, but results in low conversion rates when people find that it’s not exactly what they are looking for.