There is an issue in the MongoDB PHP driver with the way it establishes connections to MongoDB servers. This issue only happens on servers that use more than 1024 file descriptors. This means that on our shared hosting servers it is possible that you may not be able to connect to MongoDB servers. We are aware of the issue and we are working to provide a solution.
In the mean time, if you need MongoDB, you can get a VPS and you will be able to connect to MongoDB with no problems.