I provide online lead generation solutions to SMEs & business owners and from what I've come across, the results from integration with social media sites are encouraging, especially with the awareness and participation gaining popularity not only among individuals but among businesses as well.
1 angle you can look at it, is that as more businesses are connected through these avenues, it means that more individuals are able to get updated more easily by being "followers" of the various groups, business, activities, movements etc.
This is a big leap from the past where businesses were not so integrated with social sites and the latter remained largely social in nature.
In terms of effectiveness, you can gauage from your analytics how much referrals you get from such sites.
The challenge still comes in your site & social media effort & determining a ranking on your social media platform, like how google or alexa ranks a website.
For myself, I use a dashboard that helps me keep track & a record of the performance of any website (including competitor's website
) by the various search engines based on a list of keywords.
It helps me very much in managing and monitoring changes in website rankings over time, to help me benchmark the effectiveness of my marketing effort, and not just the increase in visitor traffic.