As both a user and a developer, social logins superficially seem amazing. Someone else is handling your data security, the login process is streamlined to a single button (provided you're logged in to the social provider), and integration is typically a breeze. However, those same benefits terrify me the most, and here's why.
Goodbye Yellow Brick Road
Services shutdown, or are otherwise rendered inoperable either temporarily or permanently by a variety of factors. While this is unlikely to affect services at the scale of Facebook, Github, Twitter, and the like, it can happen. In the past month, we've seen Orkut and Earbits shut down, albeit Earbits subsequently relaunched after two days. These services aren't at the same scale that Facebook and Twitter are, and Orkut's a Google owned property, but these are hardly inconsequential services. Myspace was once the pinnacle of social networks, and after its fall from grace, it has since pivoted to be a music oriented site. This begs the question, what happens if Facebook, or Twitter, or any service that provides social login capabilites shuts down, or is acquired, or even just removes its social login feature as a whole? Will they provide an alternative? Will they just fade away, leaving developers scrambling to find a solution to allow users to recover their socially-bound accounts? Or will the burdens fall to the users to start from scratch, or if they're lucky, wait for the developers to find a way to effectively merge the two accounts.
The Data Monster
Like it or not, the majoritiy of free services are in the business of data. Every bit of information fed into their aggregator increases your value to them. So what happens when you click on that "Log In With Facebook" button on... Oh, let's say eHarmony, or some other dating sites. Congratulations! Facebook just learned you visit dating sites, and will likely spit that data back out in the form of targeted content. Of course, this is already being done via "Like" boxes and tracking cookies, etc, but it's yet another vector for data collection.
I rarely use Facebook. I'm on it, but I don't necessarily like using it. So when I stumble upon an amazing site that I'm just zipping around on like a digital kid in a binary candy store, I'm probably going to want to dig more deeply into it, and 99% of the time, that means I'm going to want to sign up. So when I click that "Sign Up" button, and see a single button staring me in the face, saying "Log In With Facebook", I get understandably discouraged. I don't want to connect my account for a site to a service I rarely use. Not to mention, when given the option, users are more likely to take the traditional login route, so why force users to something that, under normal use, would barely pull in 5% of conversions?
To sum it up, social logins have their merits, and I use them shamelessly on some sites, but I beg of you, please at least give me the choice.