At the end of November, for no apparent reason, Twitter's stock went on a massive run. It hit $73.30 on December 26, an 88% increase from its low point on November 28th.
It's fallen back to $66 after a raft of downgrades from analysts. The primary reason analysts are downgrading Twitter? They don't think its valuation makes any sense. As you can see in this chart, using data from Morgan Stanley, shows that Twitter's valuation is sky high compared to its peer group.
Now, before you go and say, or do, something rash, keep in mind this doesn't necessarily mean Twitter's stock is going to crash. Some companies can defy the laws of valuation. (See: Amazon.)
