DynamoDB: searching with primary key or range key, which way is faster?

Question

To those familiar with DynamoDB, I'm trying to make a table where items only have 2 entries. For example, . Most of the time I'll be searching the user_id with a facebook_id on hand.

Amazon's DynamoDB only provides the ability to search based on 2 keys, the primary key and the range key. I have 2 options; either making the user_id or the facebook_id the primary key. Primary and Range keys differ on the matching algorithm used.

I'm just curious which way is more efficient in this case? Searching with the primary key, or searching with the range key, or is there really no difference?

Could we add DynamoDB as a tag? As DynamoDB is fairly new, I'll not be surprised if no-one knows the answer.