nobody can guarantee you to be on the first page of google 24/7...
the main reason to use google is that more than 75% (yahoo holds 7% and bing (bing/live/msn) holds around 4%) of all web searches go through google (in some form), which means for you, there is a lot of traffic that could potentially see your advertising.
the PROBLEM with google is people who dont know how to use computers, people who type in "www.firstpagegooglenow.com" or "gmail.com" into the google search bar, rather than the browsers address bar. This means that the vast majority of google searches are simply people typing in the wrong box.
though, if we count that in to the numbers, there are still millions upon millions of users, searching for valid things via google every day and you need your business to be found on there.
other search engines such as yahoo and bing (also known as live search, and msn search) also have millions of searches ran through their engines every day, and also run their own set of rules on how they index sites and how sites are displayed, so you will need to see where the majority of your hits are coming from now, and target that search engine.
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