[{provider:"Facebook",timestamp:"1371798000000",name:"page_storytellers",value:20871},{provider:"Facebook",timestamp:"1371798000000",name:"page_fans",value:1291509},{provider:"Facebook",timestamp:"1371798000000",name:"page_fan_adds",value:2829},{provider:"Google",timestamp:"1371798000000",name:"page_fan_adds",value:1000}]

db.collection.aggregate([{$group:{_id:{provider:"$provider",timestamp:"$timestamp"},items:{$addToSet:{k:"$name",v:"$value"}}}},{$project:{tmp:{$arrayToObject:"$items"}}},{$addFields:{"tmp.provider":"$_id.provider","tmp.timestamp":"$_id.timestamp"}},{$replaceRoot:{newRoot:"$tmp"}}])