Simply put, I don’t do site messages. If you sent me a site message, I will reply to your message (unless scam), but I will also include my email address. If you insist on site messages only, then I won’t be the person you would want to communicate with.
PenpalWorld’s free service limits the number of site messages you can write. They also don’t allow email addresses in the profile – forcing people to use the site message. I am able to send my email when I write a message, or reply and that is what I would do.