1. A method of providing a custom action for post in an online social network, the method comprising: transmitting, from a server to a client machine, data implementing a user interface component for display at the client machine in accordance with first computing programming language instructions provided by a first entity: the user interface component displays at least one feed item record authored by a user and a plurality of responsive posts in a thread about the feed item record, each post of the plurality of responsive posts having a feed item ID and being posted by a user with information about the feed item record, and each post of the plurality of responsive posts contains a custom action activation mechanism indexed by the feed item ID, wherein the custom action activation mechanism is customized based on a state of the post indexed by the feed item ID and is customizable with second computer programming language instructions provided by a second entity; receiving a message transmitted from the client machine to the server, the message indicating detection of a custom action activation event generated responsive to activation of the custom action activation mechanism associated with a first one of the responsive posts; and performing the custom action at the server in response to receiving the message: the custom action modifying data related to the first responsive post at the server, and the custom action being performed in accordance with the second computer programming language instructions provided by the second entity.