Most active monthly users ?

[mikeinjersey]

I'd like to award my members who are most active during every month, with a seperate announcement post. along with their username and website link of their choice.

So, once a month i'd make an announcement post congratulating the top 15 members.

But how can I find out who are the 15 most active (online) per month? This would only include logins and duration...nothing to do with Posts, etc.

thanks much