This post is sufficient for a court to reject any claims of protections based on USC § 2301.
The forum isn't based in the US.
Legality aside, decency would suggest you should put a notice on the "private message" page stating that the messages are not private and may be read by moderators. As it stands it gives the impression it is only between the two involved parties.
Not disputing your right to read the messages, but it needs to be clear to people that this is the case when they go to send a message.
As I pointed out earlier in the thread, any electronic communication that isn't encrypted
will be read by a 3rd party at some point. Assuming otherwise is like thinking the message on the postcard you sent Grandma is private, too.