In addition to using photos from your computer or mobile devices, Collage.com has made it easy to add images from Facebook and Instagram.
All of Collage.com's upload options can be found to the left of the project by clicking (this option will be below the project for mobile devices and under "Add Content" for books and calendars).
When adding Facebook and Instagram photos, you'll be asked to log into your account and to give Collage.com access to your photos. Only the first 200 of your publicly shared images will be available to add to your project (30 for Instagram). If you don't see a photo you want to use, it's likely that the privacy settings are strictly set or your images exceed those caps.
If you ever decide you want to revoke Collage.com's access to your information and photos, you can follow the directions here to remove access.
Note: you can only access your own Facebook images. You cannot access your friend's photos.