The FANG stocks have together shed more than $200 billion in market value since mid-March, when Facebook plunged after acknowledging its data-privacy issues. Facebook, Amazon and Alphabet are still off about 9% or more each over that time, compared with a 4.1% drop for Netflix.

But investors remain generally optimistic about the prospects for Amazon and Netflix as those companies continue to upend the business segments in which they operate—retail for Amazon and media for Netflix. And Alphabet’s diversified operations, from advertising to search to tech hardware, make it a dominant force in many fields.

Mr. Slingerlend says he doesn’t plan to significantly reduce his fund’s holdings in Alphabet and Amazon and expects them to maintain their heady growth paths, even if lawmakers push for more regulation of tech companies. Investors who group those tech giants with Facebook are missing the intricacies of each business, he said, dismissing President Donald Trump’s criticism of Amazon’s tax treatment and its relationship with the U.S. Postal

Many actively managed funds have a higher exposure to the FANG stocks than to the broader tech and internet segments, Bank of America Merrill Lynch said in a recent report, with Alphabet and Amazon among the most crowded stocks. Those same investments helped fuel the funds’ gains last year when all four stocks notched double-digit-percentage increases.

But the FANG stocks have lost some of their influence over the broader market. Amazon and Netflix, which are up 23% and 60%, respectively, for the year, accounted for more than 30% of the S&P 500’s 2018 gain at one point in February, according to S&P Dow Jones Indices. That contribution slipped to about 24% as of April 6. Facebook, meanwhile, is among the biggest drags on the index, overshadowing the declines of even General Electric Co.

“These companies are of a size and have an amount of influence now that, going forward, are going to have to contend with these sorts of challenges,” T. Rowe’s Mr. Sharps said of data-privacy concerns and the potential for regulation, though his firm hasn’t significantly reduced its exposure to FANG stocks.

The declines in the FANG stocks last month wiped out nearly half of the year-to-date gains of some of the best performing actively managed funds, including Mr. Slingerlend’s Janus fund, according to a ranking of funds by data provider Morningstar Inc.

That fund shed 5.2% from mid-March through Monday, paring its gain this year to 7.5%. Meanwhile, the Morgan Stanley Institutional Fund Growth Portfolio, which has positions in Facebook, Amazon and Alphabet, fell 5.8% over the same period to cut its year-to-date gain to 10%.

In comparison, about 70% of growth funds that focus on large-cap stocks like Facebook, Google and Amazon outpaced the S&P 500 over the first three months of the year to return 3.1%, compared with 1.9% for growth stocks in the broader index, according to the Bank of America Merrill Lynch data.

While March proved to be rough, 81% of those managers were able to post better returns than the negative performance of growth stocks in the S&P 500.

Even an exchange-traded fund that touts its FANG exposure and aims to invest in modern tech companies has soured on Facebook. The AdvisorShares New Tech & Media exchange-traded fund sold out of its Facebook position shortly before the data mishap was disclosed, said Scott Freeze, chief investment officer of Sabretooth Advisors, which manages the fund.

Instead, Mr. Freeze is telling current and prospective investors that his fund aims to find the next FANG stocks that are set to expand at rates similar to how the original quartet performed in prior years, such as payment-processing company Square Inc., a tech stock that is up 36% this year.

“FANG is not a set of four companies. It’s an idea,” Mr. Freeze said. “There are FANG stocks every generation, every decade. Nothing stays on top forever.”

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