NewsYelp reaches 100 million unique visitors, but trouble may be brewingThis article was originally published onBy Sean O'Neil | February 7, 2013 Yesterday Yelp, an online service that lists and reviews local businesses, announced its January traffic numbers. More than 100 million unique visitors had came to its desktop and mobile site in single month—a record.The number from Google Analytics represents, roughly, growth of about 30% year over year. It excludes the 9.4 million unique mobile devices that used the Yelp mobile app in January alone.The statistic also excludes traffic to Qype, a platform in Europe that Yelp bought in October for Euro 18.6m in cash as well as 970,000 in shares.The company put together an infographic to trumpet its success and to explain who its users are, breaking out the demographic details. SEE THE FULL SIZE INFOGRAPHIC BY CLICKING, HERE.One key statistic: Only 4% of businesses reviewed are in travel. About 21% are in restaurants, and the rest local services and activities.Trouble may be brewingYelp also released its quarterly earnings, and the news wasn't all rosy. It missed analysts' bottom-line estimates.About 70% of Yelp’s revenues come from ads by local businesses listed on its site. To keep the ad growth rising, its sales and marketing expenses have continued to climb as well.Other dangers on the horizon include:TripAdvisor will compete with Yelp as TripAdvisor introduced in beta a new Facebook app, Local Picks.Facebook is testing Graph Search as a tool for local search.On the bright side, Yelp has expanded its London office and may be able to leverage Qype to full advantage in 2013.In the past few months, Yelp also launched in Poland and Turkey, bringing its combined Yelp/Qype kingdom to 20 countries.Yelp has also benefited from its partnership with Apple. The maps introduced by Apple in September are seeded with Yelp content.And overall, its IPO flotation in March was much more enduringly successful than similar-category site Groupon's, to say the least.