The iPhone 4S has significantly boosted the fortunes for Apple (AAPL), with the company's stock price surpassing $500 for the first time. That's up from $400 last October, when the phone was launched.

Not only has the iPhone 4S helped Apple become the No. 1 global smartphone maker, but it also narrowed the gap with Google's (GOOG) Android operating system in the U.S. According to a BGR report citing StatCounter research, Apple has a 59% share of mobile Web browsing usage in North America. Globally, Apple has a market share of 30%, with Nokia (NOK) leading this market with 40% share. Our $550 price estimate for Apple's stock is about 20% above the current market price.

Mobile usage depends on smartphone popularity

Apple's massive share in mobile web usage in the U.S. is a direct result of the iPhone's popularity. According to a NPD report, the iPhone 4S, iPhone 4 and iPhone 3GS took the top three spots in the U.S. smartphone market last quarter, and Apple's iOS operating system, with a market share of 43%, narrowed the gap with Android's 48% share.

Apple is now the leading global smartphone player, recently overtaking Samsung to gain the No. 1 spot with a market share of 24% in the fourth quarter. According to the StatCounter study, Nokia is still the leading player in the global mobile Web usage.

Although Nokia is no longer a potent player in the smartphone market, it still leads the overall mobile phone market due to its large presence in the emerging markets like India. One can use web on many non-smartphones as well, which is why Nokia's share in mobile web usage is higher than Apple's.