Facebook's pain isn't over yet, according to one top technician who just noted a troubling development in the charts.

Facebook shares continued a slide Tuesday in the wake of a security breach that the company revealed on Friday and said compromised around 50 million users' data. It was trading down 1.8 percent at around $159 a share late Tuesday morning.

The skid has brought the stock's 2018 losses to 9.5 percent. The stock has been badly underperforming the broader market for quite some time, and the bottom isn't in yet, according to Craig Johnson, chief market technician at Piper Jaffray.

"We've broken the up-trend support line that's been intact since 2013, and from my perspective, it looks like the shares are destined to come back and test that $150 level. I'd just note, that's the absolute performance. On a relative basis, Facebook shares have been underperforming the S&P 500 for now 18 months. From my perspective, this is a stock that's got more downside to it, and $150 would be your first are of support," Johnson said Monday on CNBC's "Trading Nation."

Johnson said he would not step in to buy Facebook shares because the stock's performance relative to the S&P 500 reached a fresh "lower low" on the chart, a signal that concerns him.

From a fundamental viewpoint, the stock may suffer in the short term given its various problems about privacy, but the future of Instagram is one bright spot for the company, said Boris Schlossberg, managing director of foreign exchange strategy at BK Asset Management.

"At this point, I think Facebook is probably a cautious buy. I would be a seller of the $150 puts to see if I can get in at that level," Schlossberg said, suggesting a bearish options trade on the stock.

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