To explain the expression you have to put it on a context. Normally when you are friends with someone, you also support them, listen to them and at some instance, you agree with their way of living or thinking. If you are in agreement with everybody, no matter what their thinking or point of view is, it means you are really not true to anyone, which means you are not friends with anyone, you just go along. You can be "friendly" but not simpathethic with everybody because they'll always think differently. That is what the expression actually means and I do agree with it.

No, I don't agree. You could be friend to all in a general sense,yet have some special close friends who you trust the most.

Then he is a so-called, "yes man". If I were Mark Sucker-berg, I sure wouldn't, though.

No I don't. You can be friendly and helpful to a lot of people.

Yes, I agree with that. A friend to all is somebody who is not sincere. Sure, you can be friendly and empathetic to everybody (as an acquaintance, for example), but you can't possibly get along with everybody else in the world, let alone establish a meaningful and lasting friendship with just anybody. Obviously, because people have different opinions and personalities, many of which undoubtedly will conflict with yours. The idea that every person in the world must be compatible with you is ridiculous. Don't feel bad, what matters is that you cherish those close friends you have, and welcome new friendships!