"There's a few disturbing trends you cannot ignore. First one, you saw a recent study by SimilarWeb that discussed the actual use of social media is declining," Left said Wednesday.
He added, "Snapchat is getting good mind share [in the millennials age group]."
The short seller also expressed skepticism that Facebook, with a market cap of more than $320 billion, will be able to come up with new growth initiatives that will "move the needle."
"As a stock this a great time to sell," he said.
Left in October correctly called the decline in Valeant Pharmaceuticals. Citron Research, the firm he leads, is one of the few short selling research shops that consistently moves markets on newly released reports.
Shares of Facebook were down 1.5 percent for the week as of midday Wednesday.