AVATARZ is a lovely 3D library of diverse avatars for any of your projects. You can use them in your products or for marketing purposes (social media posts, landing pages, blogs, newsletters, etc.).
If you're a designer or developer looking for Facebook resources and templates, now you have an official place to find out this things. Facebook has released a library to help us with a lot of stuffs, such as GUI's, devices, articles and much more.
A collection of free resources made by designers at Facebook.
Take a look at design.facebook.com, there you will find links to Medium with great articles from Facebook designers, to Dribbble with an amazing collection of free resources, GUI templates for Sketch and Photoshop and several devices templates to feature a new app design. This is some of things available there!