Unfortunately, Facebook has denied access to third party apps from gathering information on their users. Facebook has updated their security and now requires all third party apps to submit a request and be licensed with Facebook's approval. This ensures that the information of Facebook user's is private to other apps.
Prior to this update, YouMail and other third party apps could easily link to Facebook and gather information, in this case contacts and pictures/avatars. As many users have noticed, a lot of third party apps give access to sign in with their Facebook account. The Facebook update implemented that features, which adds more security to the profiles of Facebook users.
YouMail is currently working on being able to access Facebook again. The approval process does take some time, but YouMail users will be informed once this feature is active.