mongod doesn't load automatically the default config file; if I run

mongod --repair

it prints

Wed Jun 22 17:33:08 [initandlisten] MongoDB starting : pid=3878 port=27017 dbpath=/data/db/ 64-bit

when in /etc/mongodb.conf I have

dbpath=/var/lib/mongodb

While, if I run

mongod -f /etc/mongodb.conf

the daemon starts correctly and in /var/log/mongodb/mongodb.log I have:

Wed Jun 22 17:37:03 [initandlisten] MongoDB starting : pid=3905 port=27017 dbpath=/var/lib/mongodb 64-bit