Oppenheimer analyst Jason Helfstein downgraded Twitter to Underperform, his firm's equivalent of a sell rating, with a price target of $17.

Twitter closed Friday up 21%, or $3.99, to $22.62 following press reports that the company is moving towards a sale.

A media company is the most likely acquirer of Twitter and would not pay meaningfully more than the valuation implied by our price target, Helfstein tells investors in a research note. He cites slowing user growth, poor product implementation, decreasing user engagement, inferior advertising technology, platform safety issues and competition as the basis for his $17 price target. Helfstein also sees no meaningful increase in engagement on Twitter from the Olympics or the NFL games, which he believes further diminishes the probability of a takeover. The analyst thinks Twitter is overvalued compared to its peers.