Is it ok to ask a professor if he/she is taking a leave of absence I am currently deciding between graduate schools for PhD programs and I am concerned that a potential advisor might be planning to take a leave of absence to start a new company. Is it socially acceptable to ask the professor if he/she intends to take a leave of absence?

Doing research with a professor that is on leave would be difficult particularly during the early years of graduate student life.
You should ask whether they would be willing to take on PhD students, and, if yes, meet regularly. You suspect correctly that starting a new company does not make it likely for him to have much time the next years, so even if he says (means) yes, it may not mean that he can act on his promise, so handle with care.