Amazon.com shares hit all-time highs Friday above the psychologically key $800 level, making the online merchant one of the five largest U.S. companies by market capitalization.
The growth of Amazon's stock is emblematic of a gradual rise among tech or technology-related stocks that are in the . Such companies now dominate the top of the index.
"The reason why these stocks are there (is) in an economy that's hardly growing, growth stocks demand a high premium," said Bruce Bittles, chief investment strategist at Baird.
Technology stocks are considered growth stocks, which move sharply on momentum, versus so-called value stocks that are less volatile and generally represent industries more connected to economic fundamentals such as oil prices.
The shift toward tech's dominance in the U.S. stock market has picked up speed in the last few months. As recently as May, Exxon Mobil had the fourth-largest market cap among U.S.-based companies. By Friday, the energy giant had fallen to seventh place with a market cap of less than $350 billion.
U.S. oil prices have contributed to Exxon's decline, falling more than 10 percent from a peak of $51.23 a barrel in June to $44.48 a barrel Friday.
Meanwhile, Amazon was in sixth place in May, but had climbed to fourth place with a market cap of about $381 billion on Friday.
"Over time, the business cycle changes and favors different sectors of the market," Bittles said.
Apple remains the leader with a market cap of around $610 billion. The iPhone maker's stock first surpassed Exxon Mobil to take first place in 2012, and has solidified that position since 2013.
While Amazon technically falls into the consumer discretionary sector of the S&P 500, the four other names at the top of the list of largest market-cap stocks all fall into the technology sector.
With that kind of sector domination, "the U.S. moves like one giant tech stock," said Jeff Kleintop, chief global investment strategist at Charles Schwab. He pointed out that the U.S. stock market overall closely tracks global tech stock performance.
Source: Charles Schwab
— CNBC's Gina Francolla contributed to this report.