We aren't really a primary source of information to cite, but to cite a website, you'd want to include the name of the site, the URL of the page you are referring to, and the date it was accessed. It's also a good idea to print out what the page looked like at the time you cited it, in case it changes.
However, from what you're describing, it sounds less like you really want to cite PF, and more that the individuals you had a discussion with helped you develop your ideas. In that case, if it really made a difference to your work with ideas you would not have otherwise thought of on your own, including those individuals in the acknowledgments section would be most appropriate. I would recommend contacting them by PM and letting them know you would like to acknowledge them and ask how to do so (they may prefer to remain anonymous and pass on the acknowledgment, in which case, you are no longer obligated to mention their assistance, or they may share their real names to be included). In such a case, you need not cite PF at all...we're just the means for your communication, not the source of the ideas themselves.