From Facebook Developers Wiki

[edit] Description

A container for one or more fb:editor-button tags, which are rendered next to each other with some space between each button.

[edit] Examples

<fb:editor-buttonset> <fb:editor-button value="Add"/> <fb:editor-button value="Recommend"/> <fb:editor-cancel /> </fb:editor-buttonset>

[edit] Notes

This tag must contain at least one fb:editor-button as a child.