I'm building a desktop/mobile app and in addition to the standard login I wanted to provide a way for users to be able to login to from services such as Google, Facebook, etc. The difficult part in doing so is choosing the right services to include according to my target demographic. I've seen many sites include just about every service you can think of, creating overcrowded an interface of unwanted options. Are there any resources out there that would help me make this decision by providing such statistical data? Also is there a limit to how many services you should provide to login?
The exact answer depends on your users (who are they and what services are the likely to be signed up for?) and your use case (you wouldn't want Facebook login on a business site, for example).
If you want to research this, it is easy to find demographic information for social media sites. For example, take a look at this PewResearch report.
Ultimately, though, you are probably overthinking this. Just a few common options (like Google and Facebook) will cover most users, and there is always the standard log-in to fall back on. I agree that you wouldn't want too many--mainly because of complexity of implementation and diminishing returns from adding more options.
I agree with dan1111.
From a product standpoint you will also want to think about why you are using these services. For ex: Social products often will create their own authentication-—where they can add value by creating their own social graph.