Now, don't take this as gospel but I find the best times to log on are between 10:30pm and 2:30am at the moment. There is usually a constant flow of messages between those times. It is 2:35am in the morning at the moment and there are currently messages available. Late afternoon can be a good time as well but it is very hit or miss.
Click on the username and then 'Start Conversation' and that's how you privately PM someone