Measuring has always been one of the hot potatos in the social media scene. It seems some people still find it hard to believe that social media actually pays off. Weird stuff... :)
In measuring, social media is sometimes (falsely) equated with banners, search engine marketing and direct digital marketing. Thus, with a lack of better key performance indicators (KPI), the same ones are copied and fancount/followercount is added. The latter often since that's a number we can compare with colleagues. Fancount is important, don't get me wrong, but as a sole measurement it is false. Hear me out here: it's more than "not enough". It's "false". It points you in the wrong direction. A similar mistake would be evaluating Apple as a company by how many tons of metal it purchases.
I do underline that actions require KPIs. Your webpage, office building, and Facebook Page all require measuring. Indicators are also the right way to go. I have not, and probably will not see a valid calculation of "ROI of a Facebook fan-page". If you are doing it right, you cannot find out the ROI. (not in under 20 years anyway) Sorry.
The KPI's of a Facebook Page are somewhat comparable to the KPI of our office building and surroundings. They sure are important, but showing where the money is sure as hell ain't easy. However, you can and should keep track of indicators: are the people in your neighborhood glad your office is right next to them? Do random people sit on the benches next to your office building? Do your employees smile as they open the office door in the morning? There's some hardcore measurements for you.
And what does fancount as a sole KPI tell us? It does seem to boost the egos of marketers, and it always says more about the company's line of business and natural environment.