(Updates to after market close)
By Caroline Valetkevitch
NEW YORK, Sept 24 (Reuters) - The new S&P 500 communication services sector, which includes such high-profile names as Facebook Inc, Alphabet Inc and Netflix Inc, made its debut on Monday with a small gain, after the largest-ever overhaul of Wall Street's broad business sectors.
Under the shakeup of the Global Industry Classification Standard (GICS), those stocks and others were moved out of the technology and consumer discretionary sectors into the new group - a bulked-up version of the former telecommunications sector.
The reshuffle appeared to happen without any disruptions after investors reorganized their positions at the end of trading on Friday, which was the highest-volume trading session since Feb. 9, according to Thomson Reuters data. A big chunk of the volume came in the last 15 minutes of the session.
"It's testament to the strength and transparency of U.S. equity markets that it was smooth as silk, that you can take all these stocks and reclassify them into a new sector and not disrupt trading," said Michael Antonelli, managing director, institutional sales trading at Robert W. Baird in Milwaukee.
"It was like somebody repainted a bedroom overnight. It went from blue to purple with no disruption."
The communication sector ended the day up 0.2 percent, compared with a 0.4 percent drop for the broader S&P 500.
The biggest boosts to the index on Monday came from Facebook, up 1.5 percent; Netflix, up 2.3 percent; Walt Disney Co, up 2.1 percent; and Alphabet, up 0.6 percent.
The biggest drag was Comcast Corp, which fell 6.0 percent after news it beat Rupert Murdoch's Twenty-First Century Fox Inc in the battle to buy Sky Plc on Saturday.
Twitter Inc, which also moved to the new communication sector, was up 0.4 percent, while Verizon Communications Inc fell 1.6 percent, CenturyLink Inc was down 0.2 percent and AT&T Inc gained 0.4 percent.
S&P Dow Jones Indices and MSCI had maintained the widely used industry classification system since 1999, and the reshuffling was meant to reflect how the tech, media and consumer industries have evolved.
"It was a needed change... and it looks like the transition was fairly smooth for these companies," said J. Bryant Evans, investment advisor and portfolio manager at Cozad Asset Management, in Champaign, Illinois.
Apple Inc, which remained in the S&P information technology index, rose 1.4 percent, while Amazon.com Inc, which stayed in the S&P consumer discretionary index, climbed 1 percent.
The Communication Services Select Sector SPDR Fund, which State Street Corp launched in June to track the new sector, was flat on the day.
Replacing the telecom sector - which was about 2 percent of the entire S&P 500 - the communications index has a roughly 11 percent weighting under its new communication services tag.
The weighting of the technology index, fell to about 20 percent, from 26 percent, while consumer discretionary dropped to about 11 percent, from 13 percent.
The communication services fund could continue to attract new money in coming weeks as investors seek access to stocks that dropped out of the other indexes, strategists said.
At 18.7 times forward earnings estimates, the expanded communication services sector carries a higher valuation than the prior telecom sector, whose forward P/E was just 10.5 times, according to Thomson Reuters data.
The reworked information technology sector carries a forward P/E of 18.4 times, based on the data. The tech sector's prior P/E was 19.5 times.
Reporting by Caroline Valetkevitch; additional reporting by Trevor Hunnicutt and Sinéad Carew; editing by Alden Bentley, Nick Zieminski and Lisa Shumaker