The formula for success - literally. In ad-supported sites, the total number of page views determines the ad inventory you have to sell and thus, the revenue potential of your site. The formula for page views (put in a way you can memorize) is people X checking X clicking. Put more fully, the number of unique visitors each month X the number of times they visit per month X the number of pages they view per visit - equals the number of pages you serve each month.
Checking and clicking matter more than people. Here's the secret of the formula above: increasing checking and clicking increases your page views far more than adding users. For example, in January 2008, MySpace had 59M users who visited on average 11 times each month and viewed 43 pages per visit. Adding 1 more user increased their total page views by 473. That ain't much. But adding 1 more visit by all of their members would increase their page views by 2.5 billion! And if their members viewed just 1 more page each time they visited, it would increase their page views by 650 million - which is more than Facebook, Classmates, Evite and LinkedIn combined!
The simple, powerful psychological way to increase checking and clicking. Psychologists like B.F. Skinner have known since the 1930s how to increase checking and clicking. Because the web didn't exist yet, Skinner applied his lessons to things like slot-machines and fishing. But the application to websites is even clearer. Let me translate the phrasing he might use into language you'll like better:
Skinner would say"Bolster behavior against extinction with a variable interval reinforcement schedule." You read that as: People will repeatedly visit your site if there is an unpredictable amount of time before cool content appears. For example, we don't know how long it will be before a new email arrives, before someone posts a new comment on our Facebook profile, before there's a new bid on our Ebay item, before our movie is available on Netflix, or before breaking news appears on CNN.com. This simple fact makes us check these sites repeatedly. And it makes us continue to check even after longer and longer periods with nothing new. That's the nature of frequency. If your site doesn't already have a feature that operates on a "variable interval" schedule, build one.
Skinner would say "Increase the rate of behavior with a variable ratio reinforcement schedule." You read that as: People will view more pages if there is an unpredictable number of clicks before cool content appears. For example, we don't know how many YouTube videos we will need to watch before we see a really funny one, or how many old friends' profiles we need to read on Facebook until will get some juicy gossip, or how many times we need to play Scrabulous on Facebook until we win, or how many people we need to rate on HotOrNot before we see someone really attractive. So we click on. That's stickiness. Again, if your site doesn't already have a "variable ratio" feature, build one.

