Your pages need to have the Open Graph meta tags required for social networks to parse the needed information & display it correctly. If you’re looking for a simple, straightforward solution to this, you should check out Complete Open Graph, a plugin I wrote to handle it. Much less bloat than something like Yoast, because instead of trying to do all the things, it focuses on doing one thing well — Open Graph management. Activate it, and it’ll work right out of the box.