As a long time lurker and recent joiner-upper I wouldn't recommend doing anything. If I found out numbers were being delayed for lurkers while I was a lurker I'd probably have ditched the site and moved on.
I think having high visitor numbers is a testament to the quality of the site and the information it provides. People are preferentially coming here to find out what's up, it should be a source of pride for the people running the site and it should make the long time residents pleased. The reason to join up is to join in conversation, but if people are only interested in seeing the numbers and not particularly interested in connecting with folks then why should they be compelled to join?
At the end of the day it is the quality of the discourse which encourages lukers to become members.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
