It’s still unbelievable how powerful Google continues to be.  If you need to turn on meaningful traffic volume for a Web site, they’re the only game.  And properties that depend on Google for a majority of traffic have a number of risk exposures.

For SEO (organic) traffic, there’s always a chance Google changes the search ranking rules.  Stories abound about companies ranked #3 (for some keyword) waking up some morning to find themselves ranked #11 and traffic down 80%.  Google is continually battling search engine spam, and even if you have quality content, you could easily be collateral damage in an algorithm change. 

I always think of SEO traffic as “bonus icing” — great if you can get it, but don’t depend on it.

For SEM (paid keyword) traffic, there are so many ways Google can (and does) manipulate the system to their advantage.  AdWords is like playing poker, but with the house as one of the players, and with some of the rules marked “secret”.   If you don’t believe me, read about Google’s infamous “quality score“, and then read this story about a company that hit the wall when Google changed the rules.

For SEM, I always assume:  (a) you can’t afford to build your site entirely with paid traffic — you have to find other sources, (b) CPC prices trend up over time, not down, and (c) prices can change quickly.