An exchange-traded fund that tracks a group of social-media companies fell on Monday, as Facebook came under heavy selling pressure for how it has managed user data. The Global X Social Media ETF SOCL, -2.01% lost 1.3% in what was set to be its second straight daily decline. Facebook FB, -6.77% shed 4.9% and was on track for its biggest one-day decline since November 2016. The stock is the third-largest component in the fund, comprising 8.85% of the portfolio, according to FactSet data. Facebook has ignited a firestorm over how it manages third-party access to its users' information, after saying a firm with ties to the 2016 Trump campaign improperly kept member data for years despite saying it had destroyed those records. Among other social-media companies, Twitter Inc. TWTR, -1.69% fell 1.2% while Snap Inc. SNAP, -3.47% was down 1.8%. Despite the drop on the day, the ETF remains up 14.7% thus far in 2018, easily outperforming the 2.2% rise of the S&P 500. The Dow Jones Industrial Average DJIA, -1.35% fell 0.6% on Monday while the S&P 500 SPX, -1.42% was off 0.6% and the Nasdaq Composite Index COMP, -1.84% fell 0.9%.