When it comes to the number of users that there are on your forum, it is important that you are able to give them some sort of insurance that there data is safe when they sign up with you, however you also have to have the data backed up in case you have to reload it on the website if there is a crash. There is no one that will willingly sign up again. Do you keep offline records of your website as well of the members that you have in your forum as well?