Twitter. Inc. (NYSE:TWTR) shares are down about 18% since the company reported fourth-quarter results. While 116% year-over-year revenue growth for the quarter was better than expected, one lagging metric cast a shadow over the soaring revenue: slower-than-expected monthly active user growth. Twitter reported 3.9% sequential growth in monthly active users compared to 6.4% sequential growth in Q3 -- a significant slowdown for a company with such optimistic forward-looking assumptions priced into the stock.

With billions of dollars shaved off Twitter's market capitalization, is it time to buy shares?

In the video below, Fool contributor Daniel Sparks takes a look at Twitter's fourth-quarter results -- both the good and bad -- and comments on the social network's valuation. Watch the video to find out whether Daniel thinks Twitter is a buy, hold, or sell after the sell-off.