I personally had situations where people mix up mentoring with do my work. I helped lot of people, out of those only one really worked as mentor-mentee, all others are like please have access to my computer and do it for me. I think your website will address that issue, where I can simply say, this is how it works, go and try.

But, if it is FREE service, users may misuse (or) mentors may not spend enough time. There should be some benefit structure. When I say benefit, it doesn't mean monetary, it could be anything else. Look at StackOverflow, no monetary benefit, but people go there because "reputation", which in-directly helps in lot of ways. Good luck!