* Carl Icahn cautious on stocks - Reuters Summit
* Social media stocks weigh on Nasdaq, Facebook off more than 6 pct
* Indexes: Dow flat; S&P off 0.5 pct; Nasdaq off 1 pct
By Angela Moon
NEW YORK, Nov 18 (Reuters) - U.S. stocks hit session lows in late afternoon trade on Monday with the Dow turning negative and the Nasdaq falling more than 1 percent.
The decline came a few minutes after activist investor Carl Icahn told Reuters that he is "very cautious" on equities, and that the market could easily have a "big drop."
The Dow retreated from a record high, back below the 16,000 level that it had hit for the first time in history, and the S&P 500 was now 10 points below the 1,800 level.
The Nasdaq fell nearly 1 percent, further pressured by social media and cloud-related stocks including Facebook, down more than 6 percent at $45.95.
Shares of another social media giant, Twitter Inc, dipped nearly 7 percent to $41.04.
Apple Inc shares dropped more than 1 percent to $518.94.
The Dow Jones industrial average was down 5.30 points, or 0.03 percent, at 15,956.40. The Standard & Poor's 500 Index was down 8.35 points, or 0.46 percent, at 1,789.83. The Nasdaq Composite Index was down 38.67 points, or 0.97 percent, at 3,947.30.
The S&P 500 had earlier hit 1,802.33 and the Dow touched 16,030.28, their highest levels ever. On Friday, both closed at record highs in their sixth straight week of gains.
Boeing Co shares, which rose as high as $142.00 earlier, lifting the blue chip Dow index for most of the day, were up 1.4 percent at $138.02 in late afternoon trade. The U.S. planemaker, at the Dubai Airshow on Sunday, announced commitments for a total of 259 of its new 777 jets. Worth about $100 billion at list prices, it is the largest combined order in Boeing's history.
Tesla also extended losses, down more than 11 percent at $119.83, giving up more than 20 percent for the month so far.
A number of U.S. Federal Reserve speakers offered more insights into the central bank's stimulus. The latest was Charles Plosser, president of the Philadelphia Fed, who said improved economic and labor market conditions suggest the Fed should set a fixed dollar amount on its current bond-buying program and end the program when that amount is reached.
William Dudley, the president of the Federal Reserve Bank of New York, said on Monday that he was becoming "more hopeful" about the U.S. economy.
But with intervention from the Fed likely to keep interest rates near zero for the foreseeable future, equities are expected to continue to attract yield-seeking investors, even after the Fed begins to scale back its asset purchases.