From a cultural standpoint, I would say absolutely it is acceptable. From an "inside information" standpoint, I would say absolutely not acceptable. For example, if I am interested in Company ABC and I know an employee, I would reach out to that person and ask if he/she would have time to tell me about the culture of Company ABC. I would let that person know I had recently interviewed with Company ABC and was interested in learning more should I move forward in the hiring process. If that person agreed to speak with me, I would ask questions focused on their experience within Company ABC. In reality, this would be part of the due diligence I would do on the company during my research and interview process.
On the other hand, I would not ask for or engage in any conversations around inside information such as possible internal candidate's specific information, personal questions about the hiring manager or personal questions about the person that previously was in the role I was interviewing for.
I believe it is our responsibility to gather as much information about a company/organization as possible without invading the personal information of others. There are so many resources available for an interviewee to be fully prepared to interview and accept an offer, that not only helps the interviewee but also lets the interviewer know a decision was took preparation and a well thought out plan.