If you want to be sure to keep your communications private, do not use a website or board. If the owner or mods need to get into an account or comms (say, due to a court order for example) they can. I am not naysaying JJ, but it is a power that can be bestowed, and you just never know who may have been granted such power.
Email unrelated to any board such as this is a more secure method. Of course, as with any computer based comm, you need to have a strong account password and a secured (anti-malware defending) computer. An email service such as Hushmail even encrypts the messaging between sender and receiver. The truly paranoid can also encrypt their hard drive. Also, use an email address other than the one associated with this site.
I would recommend using this board PM for messages that you do not need secured, and for verifying it is you and not an impostor that has contacted a lady by some other means.