The Rate user (reputation) button has disappeared from posts since the upgrade to 1.8.1
Go into your post_bit templates.
Check whether....
1. ACP >> Configuration >> Reputation >> Enable Reputation Functionality is set to "Yes"
2. ACP >> Templates >> Post Bit Templates >> postbit has the "{$post['button_rep']}" variable in the template..
I found the answer. The Thanks System was integrated with Reputations.