One of the best and easiest ways to get your unique visitors is to use google's web analytics - free to use. For measuring social media, see below.You can also figure out your costs and revenue per unique visitor.
"The value of a site's uniques depends entirely on what the site does. Specifically, it depends on how the uniques are monetized.
The chart below illustrates the value of a unique from several different types of web properties. Not surprisingly, Google's uniques (search) are the most valuable: $18 of revenue per monthly unique per year. Facebook's uniques (social networking), meanwhile, are worth a paltry $3.50. Twitter's uniques are worth almost nothing."
From Silicon Alley Insider, "CHART OF THE DAY: Here's How Much A Unique Visitor Is Worth".
On the other hand, measuring social media requires a different approach. "Twitter is amongst new media channels that are challenging how we communicate, with whom we communicate and perhaps most fundamentally how we (Marketers) influence people. From the Occam's Razor blog, Social Media Analytics: Twitter: Quantitative & Qualitative Metrics".