The Global X Social Media Index ETF (NasdaqGS: SOCL) has been a star performer over nearly every time horizon this. SOCL, the first ETF to focus solely on the social media sector, is up 8.7% in the past month and 48% year-to-date, making it one of the top-performing non-leveraged sector ETFs.

SOCL is also growing by leaps and bounds. Although it can still be classified as a small ETF, it had $35.5 million in assets under management as of September 20. That is up from about $20.2 million a month earlier when we highlighted SOCL as an ETF that was benefiting from the surges in other social media stocks beyond Facebook (NasdaqGM: FB). [Beyond Facebook: Other Reasons to Friend the Social Media ETF]

However, even ETFs as strong as SOCL do not move up in straight lines as evidenced by the fund’s 1.7 drop on Monday. That marked SOCL’s first two-day losing streak in over a month. Volume was heavy in the fund at nearly 489,000 shares compared with average daily turnover for the trailing three-month period of just under 56,000 shares. To be fair, SOCL has seen heavy volume up days as well. [Yes, Really: More Good News for Social Media ETF]

SOCL’s Monday woes were likely the result of a 10.1% loss for Pandora (NYSE: P). Shares of Pandora plunged after after Apple (Nasdaq: AAPL) said it brought in more than 11 million new users to its iTunes radio service. Pandora has 72 million listeners, but the competitive threat from Apple is clear and problematic for Pandora, which is SOCL’s fifth-largest holding at a weight of 6.7%.

SOCL is not a docile ETF. It has a beta of 1.24 against the S&P 500 (betas above one are generally considered high) and a standard deviation of 20.1%, according to Global X data.  That says on days when one of its top holdings drops more than 10%, SOCL is going to be vulnerable.