Yes you will have giving consent to post "things" to Facebook without seeing or approving them.
I suggest getting a 3rd party app to help. SystemPanel App, a full feature task manager, or App Detective, find every detail about an app. The first is a paid app, but there is a free version available.
Between these 2 apps you will be able find the culprit, get all the details about it, what app installed it, who made it, plus all sorts of other info, shut down the app, and to know the package name to uninstall.
Be careful when installing anything from other apps or questionable social websites. Read the fine print and examine permissions. And always make backups.
A social network service or social networking service, most often called SNS, is a medium for establishing social networks of people who share interests and/or activities.
Social networking sites allow users to share ideas, activities, events, and interests within their individual networks. Most social network services are web based and provide means for users to interact in various ways, such as e-mail and instant messaging.
Modern internet-connected always-online phones stake a lot on SNS connectivity and integration with the various services getting more popular than ever.
SNS is used for your social site widgets. It allows to post to multiple social sharing apps at once. Anything that you post on one site will be posted to your other sharing site. It can be safely disabled or uninstalled (root) without hindering Facebook or other apps.
The Play Store has several apps that can manage the SNS. I'll leave the choice for you.