You want to build a cool app to share your photos on Twitter, Googgle+, Facebook... Tackle all the 'cool' social networks. First thing you need to do is: authenticate and authorize your app for each providers.
Good news is: there is a common protocol for that. Its little name is 'OAuth2'. Each provider comes with an iOS sdk to ease the OAuth2 dance, but of course, each has its own API.
Ouch! And what you really want is use the same API for all them. Let's see how you can implement OAuth2 protocol and provide a common API across providers. Much easier than his elder brother OAuth, we will dive into OAuth2 protocol and see the challenges of implementing it from a native app perspective.