These companies talk about "visitors" per day. The challenge is to turn "visitor" into "customer" - click on an ad; buy something from the advertiser; or become a paying subscriber.
I belong to FB because I use it as a research tool and read one of the blogs. So I am a visitor - perhaps several times a day.
I probably visit Yahoo multiple times a day for stock portfolio reasons; frequently read the message boards and sometimes post. Under one single ID. I am probably counted as a visittor multiple times per day but I am a deadbeat, not a customer. That is the difference.