So much for Facebook Poke killing off Snapchat.

Facebook's new iPhone app, which copies Snapchat's self-destruct feature for messages, disappeared from the top of Apple's most downloaded apps list almost as quickly as messages disappear from the app.

Poke was released on Friday and rose to the top of Apple's App Store that same day, displacing Google Maps as the most popular free app in the U.S. The app has since dropped out of the top 10. In fact, it's not even in the top 25.

As of publication, Facebook Poke has dropped to 34th on the list of the most popular free apps.

One theory put out by Josh Constine at TechCrunch is that Poke's decline may be due to the fact that many people received new iPhones for the holidays and likely downloaded more essential apps for the device, pushing Poke lower on the list. This point isn't without merit, as some older apps like YouTube and Instagram experienced a significant uptick on Christmas Day, according to ranking data from App Annie.

However, even if this is part of the reason for the decline of Facebook poke, it's worth noting that Snapchat — the app that Facebook Poke was supposed to kill off — has actually gone up in the rankings during the same time period. Snapchat initially dropped from 6th on the list to 8th on Friday after Poke came out, according to App Annie, but it has since recovered and is now the 4th most popular free app in the App Store. Poke may have struggled during the holidays, but Snapchat certainly didn't. Perhaps users tried out Poke and then decided to download Snapchat afterwards.

What's more, Poke actually started to slip in the charts by the end of the weekend, several days before Christmas, suggesting that the app received a rush of downloads in the first 48 hours, but then quickly fizzled out. It's not too surprising that downloads for the app would be front-loaded given the promotional power of Facebook, but the speed at which the app faded suggests Poke may be more of a fad than Facebook probably hoped.

In some ways, this shouldn't come as too much of a surprise. The app has been criticized in several early reviews, including one by Mashable's own Pete Pachal, who described it as being "frustrating as hell" to use.

Image courtesy of iStockphoto, jcsmily