You can find all the available services supported by AddThis in the following pages.
AddThis – Service Codes
For example, if you want to add the Reddit button, check the code in the above link. The code is reddit. So you need to append addthis_button_ to the reddit and set it as the class. Finally, add the following line to your AddThis code snippets.
<a class="addthis_button_reddit"></a>
Done =)
Eureka! 