I've seen a lot of users on this site that don't seem to know how to properly use this feature for their social media profiles. I want to address this problem once and for all! I'll list the procedures in a step-by-step format (I think this should work for both PC users and smartphone users):
1. Reach the wall screen of your profile.
2. Above your username there should be four button, you are going to click the one that reads
3. Scroll down until you reach the Social Media Profiles section.
4. You'll see a sentence that says, "type in your username as it appears (case-sensitive) without any symbols or hyperlinks." For example, if I want to link my steam account, https://steamcommunity.com/id/Juicydeath1025/, I would just use Juicydeath1025. Also, if you don't have a custom URL and it's just numbers write just that part.
5. Hopefully, when you have reached this step your social media profiles should be working correctly.