D e m o g r a p h i c s
Why does demographics matter?
Social networks radically differ in demographic proposition and one could seem to think you're gotta agree e.g. that males are not interested as much as females in fashion (specially, a women confection), neither kids, nor cats, or some else "cute and funny staff", etc, whereas between males is somehow more fans of collective sports, technology and whatsoever.
If you target on visitors from workspace offices, freelance IT professionals, or you're distributing a desktop software, then you're not not interested in mobile visitors such as from Twitter, Pinterest and Instagram. 80% of Twitter's visitors is viewing via mobile devices. Two main reasons are associated with their age and a smaller income in the country of their origin. However, that's more likely just from the Twitter fans to follow Brands, activists, anything else that "is cool", or suited specially for people with collage education (some majority of adult Twitter users have reached a higher degree).
How to target the best demographic groups for you
Well, we already know who is where, but we need to make a research on our best readers/buyers to be able to use previews statistics. We need to know which demographic groups are mostly interested about us and who is not at all.
One of the basic solutions is to use free service from Google Analytics, that can provide very deep demographics of your website visitors as a country of their origin, gender, age of your visitors and even interests and many more... You can know what they are interested for. For example, if people comming to you browse on internet websites relative with sport and they're receiving feeds on Google plus about it, so you will get to know. This is something what you can get only from companies like Google, Facebook and Alexa, while you cannot (if you don't breach a cyber law) use any own data mining technic to find out what are doing your visitors before and after when they are not browsing your website if they're not gonna disclose it by themselves in some survey. Also you'll be aware that where they're comming from to your website, ie if they'll come from Facebook, Google, or whatsoever, then statistic received from Google gives you accurate knowledges that which one network is most worth for you.
Facebook and else media pages
You can mine some statistics about your audience from admin UI of pages managed by you. Administrative system of Facebook, or Google Plus is offering it by itself and it's absolutely free.You just need to create a page.
There's (even open source) software helping you to predict the future of your visitors.
This is an old fashion style of a data mining. Just ask them.
Fans of websites with similar content
See followers of your strongest rival. You can see who likes, shares and comments the rival's social media pages. There's an obvious evidence of fans and an option to collect the data (very simply).