Generating "leads" means turning one's users into useful contacts to create business; turning users into "leads" that is, new potential customers to work on.
The infographic below ( found at http://www.marketingtechblog.com/social-media-lead-generation/ ) shows the impact of social media on search engine results. If you have good content and share it on social media there is a chance that more people will cite our content in theirs actually increasing our rank. So while search engine optimization may be the key to bringing in "leads"-you cannot have a good rank without a solid social media presence.
Sharing content, planning events can directly generate "leads" for your company..but a solid social media presence builds authority, trust and help people decide at their moment of truth