* Weekly jobless claims data due
* GM profit beats on strong U.S. demand, smaller loss in Europe
* Facebook shares surge in premarket following Wed's results
* Futures down: S&P 8.6 pts; Dow 91 pts; Nasdaq 18.75 pts
By Angela Moon
NEW YORK, July 25 (Reuters) - U.S. stock index futures fell on Thursday as recent earnings from some bellwether shares triggered concerns, while investors worried about the impact on global demand of China's slowdown.
Investors booked profits from a recent rally that has taken the S&P 500 up nearly 18.2 percent for the year. Despite a back-to-back decline in the last two days, the index is still near its all-time high.
In heavy week of earnings, General Motors Co on Thursday posted a stronger-than-expected quarterly profit on demand in North America and cost-cutting in its struggling European business. The stock rose 2.6 percent to $38.10 in premarket trade.
Dow Chemical Co's adjusted quarterly profit jumped 16 percent, topping analysts' estimates, driven by robust demand for pesticides. The stock was up 0.7 percent at $34.60 in premarket trade.
Shares of Facebook jumped nearly 20 percent in premarket trade to $31.70, a day after the company announced earnings.
But not-so-stellar results from bellwethers like Caterpillar and AT&T on Wednesday triggered concerns about the profit outlook. Caterpillar shares were down nearly 1 percent at $82.68 and AT&T fell 0.1 percent at $35.56 in premarket trade.
"We have 50 or so S&P 500 companies reporting today. This is a lot and the results will be mixed," said Sal Arnuk, co-founder of Themis Trading, a brokerage firm that specializes in stocks.
S&P 500 futures fell 8.5 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 67 points, and Nasdaq 100 futures lost 10 points.
European shares extended losses around midday on Thursday, as mixed earnings and lingering concerns about the pace of growth in China triggered a bout of profit taking following recent sharp gains.
Chinese stocks suffered their second straight loss on Thursday despite measures from the government to spur the economy, including help for exports and railway investment. Data on Wednesday showed manufacturing in China running at an 11-month low in July.