As was suggested in https://jira.mongodb.org/browse/NODE-751 we switched to version 2.2.1.

After a while we realized that the thread monitor of the mongo process reported an alarming count of threads that have been created.
(See attachment1 for the growing count of threads)
After each restart of our nodejs application the thread count dropped down. A `db.serverStatus()` confirmed the suspicion that the huge number of threads stood in direct relation to the number of open connections on the database.

This morning we switched back to the previous version (2.1.18) as expected the thread and connection count kept stable.