Allowing people to use their social media (or other) accounts like this is common practice. It provides an easy way for people people to log in to the site if they choose to do so in this way; it's one less account that you have to worry about, as you simply need to know your social media account login in order to access this site.
Data has shown that users tend to be dissuaded from joining a site without this feature as they don't want to have to fill out yet another registration form, remember another username/password (not everyone uses a password manager), or in the case of forgetting their login info, go through the recovery process.
If you have some concern with you feature you're welcome to let me know and I will be happy to further explain.
It's entirely opt-in, too. You can simply choose to ignore the feature if you don't want to use it. The option is merely there for those who want it.