Isn't that obvious? Convenience always hat some kind of price tag, particularly a security related one.

I would have canceled my facebook account long ago if I had not chosen their login for a (unknown) number of service.

What would be a better alternative? Use same credentials everywhere? No, because it is just a matter of time it would leak out of one service. Use unique credentials for each service in local password manager? Nay, because most of us at least want to sync between desktop an mobile. Use something like Chrome's password manager? That bears similar dangers like those the article points out.