Whenever you are visiting a Friend's shared document or photo blog, at both the top and bottom of the shared document there is a row of buttons including an "Add Comment" button:

Click on it and an edit window then presents into which you can type in a comment or text message of any length, including use of any of the formatting tools at the top of the edit window. When you are finished typing your command, click "Save and Exit", which saves your comment and returns you to a list of folders and documents.

Whenever you add a comment to a share a "good share" vote is also automatically cast for that share. (learn more)