It's just "linked accounts" it's not information of profile, users are needed add the url of social same add the link to website in "Profile" tab, so how to do that?
Users can add a single URL anytime, that is the only custom field.
When someone links his Twitter account, that url (twitter) will appear in his profile. The only way to get those is linking the social service account. There won't be manual introduction of links there.