About two years ago I visited a friend in NYC who was part of a team doing a mobile shopping app. He gave me a link to download the beta app to my iPhone. To login to the app I had to use either my Facebook or Twitter account, both of which I had at that time. I asked him why I had to use either of those accounts to login and he told me that the company did not want users who did not have social media accounts. I said I thought that was a bit presumptuous, especially for a startup, but he assured me that the special login requirement would not affect customer adoption of the app. An historical note, the app never caught on and my friend moved to a different startup.
Yesterday I read a story, "Users Are Growling About Apps That Require Facebook", where they describe a dog app where you have to login with Facebook. Given that dogs are not allowed to have Facebook pages and friends (which I personally think is very unfair), the writer points out that requiring a Facebook login for a dog app is illogical. This story made me think of my friend in NYC.
If the login does not facilitate ease of use or increase the value of the app to the user, keep it as simple as possible.
Image credit: thebarkpost.com