"Yahoo has no vision. It has no purpose. It's dispensable."

Facebook may have no vision. (I actually think they do. Even if it's not very clear or savory.) But facebook certainly isn't dispensable. The analogy would be much more complete if you imagined that Yahoo was the only main email provider, emails didn't get exchanged between servers and in order to get an email to yahoo mail users you had to go to Yahoo to mail them. That's about how dispensable Facebook is to a lot of people's private lives.