Oracle has the most feature rich, relational database available. It is also significantly more expensive than its closest competitor. However, it comes with a 20%+ support/upgrade contract that is paid annually and Oracle's support has been getting worse while the best "upgrade" features are now split out as "new" products that need to be paid for. In addition, Oracle has the habit of auditing its customers as a way to extract more revenue from them.
As a result, customers are looking for alternatives and are realizing that, in many cases, they don't need many of the features that Oracle has. Microsoft SQL Server has grown in market share as a result, but now customers are finding that Microsoft's licensing practices aren't that great either. Open source options like Postgres and MySQL along with the Cloud-based databases are gaining traction in the relational space. Then there are various NoSQL and NewSQL databases that address data problems where relational databases aren't a good fit. MongoDB is one and they fit specifically within the document oriented NoSQL camp.
While it might be great marketing for an upcoming IPO, Mongo isn't an "Oracle killer" by itself. Oracle is largely doing that to itself through its arrogance and abuse of its customers. Even so, there are still transactional systems of significant scale where the Oracle database is the only real choice. And let's not forget any company that runs Oracle financials, ERP or any other Oracle application where the only offered solution is an Oracle database. Even though alternatives are increasing, growing better all the time and are taking away business that Oracle would like to have, Oracle is still very profitable (although increasingly as a result of draining experience in order to reduce labor costs) and can continue to limp along for a very long time.