Released in 2009, Angry Birds has been one of the most successful apps of all time. At one point, the game boasted 263 million active users. But while Angry Birds may have jump started apps and turned them largely into the market they are today, it hasn’t protected the company behind the game, Rovio, from investor anger. They learned this firsthand when the value of their company was halved after they reported less profits than promised.

The drop in Rovio’s value was due in large part to the fact that stockholders began a mass dump of the company’s stock following an announcement that their profit margin would be 4% lower than expected. The stock was sold so quickly and in such massive quantities that it was being traded at less than half of its listed value. This is bad news for Rovio, who only went public 5 months ago.

At the time of Rovio’s decision to go public, they had been evaluated as a billion dollar company – something known as a ‘unicorn’ in the investor community. Despite this evaluation and a recent move to enter into the esports market with Angry Birds Champions, investors have started to become uneasy about the future of the company. This should come as little surprise though.

In the investment world, ‘unicorn’ companies are supposed to be rare, but in recent years, every startup has declared itself a unicorn based off of evaluations that they themselves have done. While it’s certainly true that Rovio’s domination of the mobile market has made them millions of dollars, the question of sustainability has never fully been answered.

Pokemon Go saw a similar loss of users and profits following a decline of 80% in their user numbers. Since their last report, they had only 5 million users, 50 million less than they had a year ago, and it’s certain this number has declined since the last report.

Angry Birds was flying high for four years following the launch of their game, but since then, they have said little about active user numbers or the growth of their audience. The Angry Birds movie may have helped line their pockets some – with the movie raking in $349 million globally – but it doesn’t seem to have injected enthusiasm into the brand the way Rovio has hoped.

What might be more productive for investors and developers to consider is that mobile apps are passing trends more than lasting legacies. In many ways, mobile games are exactly like hundreds of startups that rise and fall all the time. They show up, get a lot of attention with a lot of promises, then they burn out, leaving a few millionaires in their wake and dozens of people without jobs. Maybe that isn’t the fate of Rovio, but we’d start checking to see if that horn isn’t glued on.