I have my own auth system that I use in all of my other projects. Why on earth would I send users to, and trust, a third party? That seems like an extra heada

I have my own auth system that I use in all of my other projects. Why on earth would I send users to, and trust, a third party? That seems like an extra headache. It makes sense for me to do it with facebook because it's really easy for users, and because there are benefits for the users when they do it (and for me). (These benefits are: they don't have to rebuild their social network, again)