Here’s a great infographic on demographics of social media users for the most popular sites. Really great for figuring out which site to put most of your effort and marketing budget. For example Facebook is mostly older females and Google+ is mostly younger males. If your business is making custom dolls, you probably want to be interacting with your customers via Facebook. ;-)
However, once you figure out who your market is and choose the appropriate social site, don’t ignore the others. The other sites can still provide great value in SEO for you websites and credibility to your brand and products. But focus your user interaction primarily on where you target market hangs out.
This is useful for not only niche apps but for any online business.