NEW YORK (TheStreet) -- Shares of Facebook (FB) were slightly down in early afternoon trading Wednesday as investors prepared for the social network to release its third quarter earnings after today's market close.

RBC Capital Markets analyst Mark Mahaney has a $170 price target on the stock, but sees an unlikely downside scenario in the company's third quarter numbers that could send it tumbling below $120 per share. Facebook was trading at $128.06 in the early afternoon.

"Ad revenue growth, year-over-year, it's been north of 60% the last two quarters, adjusting out for foreign exchange," Mahaney said Wednesday on CNBC's "Halftime Report." "If that were to materially crack below that - which I think is highly unlikely - but if it were, yeah, that would take the stock down below that $120 level."

Drops in monthly average user growth and engagement are also worth monitoring, Maheny added, although he clarified that significant dips in any of those metrics are unlikely.

"I don't think any of those three scenarios happens, but that's what I'm watching out for," he said.

(Facebook stock is held in Jim Cramer's charitable trust Action Alerts PLUS. See all of Cramer's holdings with a free trial.)

Separately, TheStreet Ratings objectively rated this stock according to its "risk-adjusted" total return prospect over a 12-month investment horizon. Not based on the news in any given day, the rating may differ from Jim Cramer's view or that of this articles's author. TheStreet Ratings has this to say about the recommendation:

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