Facebook, for instance, reached a valuation of $50 billion while still private, thanks in part to investments by fund companies such as T. Rowe Price, according to date from VentureSource. On an inflation-adjusted basis, that was larger than the market value of all but 14 companies in the Nasdaq at its record on March 10, 2000, according to data from Nasdaq OMX Group.
"Companies are staying private longer to develop their businesses so that when they go public they have a better chance of success out of the gate," said Michael Cuggino, portfolio manager of the Permanent Portfolio Aggressive Growth Fund.
Even if there isn't a bubble now, investors face other concerns. Chief among them: some of the largest companies in the Nasdaq are showing their age, a worry that they didn't have 15 years ago.
"What you're seeing with companies like Cisco and Qualcomm is that they have evolved to a point in their life cycle where it's very tough to show a lot of growth. That's the huge challenge now," said Skip Aylesworth, the portfolio manager of the Hennessy Technology Fund.
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Cisco said in August it would cut 6,000 jobs, at least the third workforce reduction in about as many years for a company once synonymous with the Internet boom, but which has lately struggled to sustain growth. Semiconductor maker Qualcomm has branched into markets such as medical equipment and solar panels in recent years in order to increase its growth rate.
Investors' long memories of getting burned in the last crash has weighed on the Nasdaq, which has yet to surpass its previous highs even as the broader Standard & Poor's 500 index and Dow Jones Industrial Average have repeatedly topped new peaks over the last year, fund managers say.
The rally in the shares of Apple—which has jumped nearly 68 percent over the last 12 months, including a 16.7 percent gain for the year to date—is likely to push the Nasdaq to new records, fund managers say.
"You have the largest company in the world continuing to grow at above-market rates, which is incredible," said Aylesworth, the Hennessy manager.
Apple represented slightly over 10 percent of the Nasdaq's market value as of Wednesday's close. At its record high in 2000, Apple made up less than 0.2 percent of the index. At the time, company co-founder Steve Jobs was just three years into his second go-round as its chief executive, and the company was still a year away from introducing the iPod digital music player.
Despite its growth rates, Apple still trades at a trailing price to earnings ratio of 17.3, in line with the broad S&P 500 index, and pays a dividend yield of 1.4 percent.
Apple's outsized presence in the Nasdaq—along with that of Google, a company that was still private 15 years ago and now constitutes about 4 percent of the index—is one sign that the technology sector has matured, said Phil Orlando, chief equity market strategist at Federated Investors.
"It's taken fifteen years for the Nasdaq to grow into something rational," he said.