US STOCKS-Facebook drags on Wall St; S&P 500 set to extend losing streak

* S&P 500, Nasdaq (Frankfurt: 813516 - news) on pace for 8th straight down day

* CBOE volatility index at more than four-month high

* Fitbit (Frankfurt: 5FB.F - news) swoons after forecast misses expectations

* Indexes down: Dow 0.02 pct, S&P 0.29 pct, Nasdaq 0.69 pct (Updates to late afternoon)

By Lewis Krauskopf

Nov 3 (Reuters) - Wall Street fell modestly on Thursday, with the S&P 500 headed for its longest losing streak since the 2008 financial crisis, as Facebook (NasdaqGS: FB - news) shares weighed and investors grappled with uncertainty over next week's U.S. presidential election.

Facebook shares tumbled 5.8 percent as the world's largest online social media network warned that revenue growth would slow this quarter.

The stock was the biggest drag on the S&P 500 and Nasdaq, which were both on pace for their eighth straight session of losses.

Investors have been unnerved by signs the U.S. presidential race between Democrat Hillary Clinton and Republican Donald Trump is tightening, after Clinton had until recently been thought to have a clear lead.

Two polls showed Clinton maintaining a narrow lead nationally ahead of the Nov. 8 election, echoing other polls that have shown Clinton with a slimmer lead since the re-emergence last week of a controversy over her use of a private email server while secretary of state.

"The polls have tightened and now the concern is more about what might a Trump presidency look like and the market hasn't quite priced that in," said Ernie Cecilia, chief investment officer of Bryn Mawr Trust in Bryn Mawr, Pennsylvania. "Given the fact that the election is five days away, that's what's driving near-term behavior right now."

The S&P 500 lost 6.01 points, or 0.29 percent, to 2,091.93 and the Nasdaq Composite dropped 35.25 points, or 0.69 percent, to 5,070.32.

The Dow Jones industrial average, which does not include Facebook among its components, fell 3.32 points, or 0.02 percent, to 17,956.32.

The CBOE Volatility Index, a gauge of near-term investor anxiety, climbed 9.1 percent to its highest level in more than four months.

"This kind of noise around politics is here to stay for a prolonged period of time" even beyond the election, said Phil Blancato, CEO of Ladenberg Thalmann Asset Management in New (KOSDAQ: 160550.KQ - news) York. "So what does that mean? Noise around the markets."

In a negative technical sign for the market, the combined number of 52-week lows on the NYSE and the Nasdaq significantly outpaced the number of new highs.

With (Other OTC: WWTH - news) the S&P's recent slide, the benchmark index's 2016 gain has been trimmed to 2.4 percent.

Tech stocks fell 1 percent and were the worst-performing major sector, with Facebook dragging.

Fitbit shares sank 33.9 percent after the wearable fitness device maker's revenue forecast for the holiday shopping quarter fell well below estimates.

American International Group (NYSE: AIG - news) fell 4.5 percent after the insurer's quarterly profit report.

Declining issues outnumbered advancing ones on the NYSE by a 1.17-to-1 ratio; on Nasdaq, a 1.45-to-1 ratio favored decliners.

The S&P 500 posted 2 new 52-week highs and 18 new lows; the Nasdaq Composite recorded 29 new highs and 170 new lows. (Reporting by Chuck Mikolajczak in New York and Tanya Agrawal in Bengaluru; Editing by Anil D'Silva and Nick Zieminski)