Loopback is just basically meant for creating your API, which I believe your social app will be communicating with. either by creating, updating or reading data.
Strapi is basically a headless CMS that exposes its API endpoints at the same time gives you interface to actually add your data. Strapi best use case is when you are actually creating a platform where people can only view data e.g blog, newspaper, personal website.. they are not usually supplying the data. and you don't want to be stressing yourself to actually start creating a UI or portal where you will be supplying the data.
In your scenario where it actually a social app.. most of the data is actually from the users, either by post, comment, video, pictures e.t.c. then it makes more sense to actually use Loopback... loopback is actually an amazing tool. I have played around with it before.