Really, 99% of all organisms that have existed are now extinct and about 2400 species disappear daily. Most species exist 2 to 5 million years, and--sobering thought--there's no real reason to believe our species is any different.
One reason this is true is there is a continuous, low-level rate of extinction. Recent news articles about the extinction of the Chinese white dolphin highlight one such case.
Some people may argue that this extinction was caused by humans, but competition between species has always been a factor. While it's unlikely that we'll be wiped by an evolving species of kangaroo, it's not beyond question that a "successful" bacteria or virus could wipe us out. Indeed, a species of gorilla is also in the news, threatened with extinction by the Ebola virus.
Another reason for extinction, especially the large loss of species in a brief geological period of time, is mass extinction caused by rapid climate change resulting from volcanic activity and meteor impact, such as when the dinosaurs disappeared some 65 million years ago.
The good news is about 10000 new species are discovered and described annually. Of an estimated 30 million species of animals that exist today, only 1.1 million have been described.