Facebook stock closed Thursday down 6.1% after it reported fourth-quarter earnings Wednesday evening. The drop erased more than $30 billion from the social media giant's market cap. The decline comes after Facebook reported a 51% rise in expenses compared with its total in 2018. The expenses were largely related to the company's privacy and security improvements. That coincides with a drop in the company's operating margin, which fell from 45% in 2018 to 34% in 2019. In addition to the increased expenses, Facebook also warned of advertising headwinds related to privacy and regulatory changes on the horizon, leading to slowing growth in the U.S. Facebook said privacy improvements on Apple's iPhones and Google's Android software could hurt its ability to target advertising.

In a note to investors Thursday, Pivotal Research Group reduced its rating from buy to hold on those advertising concerns. Pivotal lowered its price target to $215 from $245. "The slow down, particularly in the U.S., was far greater than we expected and sounds likely to persist," Pivotal's note reads.