Setting socketTimeoutMS as connection url parameter has no effect.

Debugging into mongo_client.js shows that the socketTimeoutMS parameter appears in the _finalOptions variable, in mongos_options.socketOptions, rs_options.socketOptions and server_options.socketOptions properties.

After that, the _finalOptions.socketTimeoutMS is set to the default value:
if(_finalOptions.socketTimeoutMS == null) _finalOptions.socketTimeoutMS = 30000;

_finalOptions.socketTimeoutMS is always undefined, because the value is in the mongos_options.socketOptions, etc.