MongoDB: How to shard based on country name

I am new to MongoDb: I am creating an small use-case , where am streaming Twitter data into my mongodb.

what I want is all the tweets from usa should go to one shrad ->shrad0000

I did shrading as below:

mongos> db.runCommand({enableSharding: "twitterdb"}) 

mongos> db.runCommand({shardCollection: "twitterdb.tweets", key: {country: 1}}) 

mongos> use twitterdb 

sh.addTagRange( "twitterdb.tweets", { country: "usa"}, "usa")

sh.addShardTag("shard0000", "usa");

sh.addTagRange( "twitterdb.tweets", { country: "usa", name: MinKey }, { country: "usa", name: MaxKey }, "usa")

But if I insert data into it... shard0000 has all countries data.

What Am I missing here? Can anyone help me out. Thanks in advance