Hello. Welcome to whatnext. If you would like to respond to a comment in public on your page, click on the little word, "comment" on the post to which you want to respond.
You can also go to their page by clicking on their name.
If you want to communicate in private, there is a link up on top of their page that says, "exchange email." It's up to them whether they feel like they want to exchange emails or use their page only.