Google (NASDAQ:GOOG) (NASDAQ:GOOGL) is still an unquestioned king in Silicon Valley, but over the last year or so there has been several reasons to believe it is losing its luster. Specifically, there are five reasons I wouldn't own it. The biggest concern I have is regarding Google's biggest business, its near $60 billion a year advertising business. Google has an average revenue per user (ARPU) well over $45 wheras Facebook's (NASDAQ:FB) is just $9 in North America and Twitter (NYSE:TWTR) and LinkedIn's (NYSE:LNKD) is far less than that. This naturally creates the question of how much upside remains for Google's advertising business when competitors are charging so much less with very effective advertising products. Furthermore, Google might soon face new competition. The Wall Street Journalreported (WSJ registration required at link) last August that Amazon.com (NASDAQ:AMZN) is prepping an advertising platform for both its own site and third-party sites. In the past, Amazon.com had used Google to sponsor ads on its site, to the tune of $1 billion worth of ads last year. Thus, Google will lose that business and gain a new competitor if Amazon.com enters the space. Read more