That's just it, I looked and looked at the UI and couldn't find out how to actually PM you. :(
Clicking on the username ("teamsnapzu") above the comment drops you into its feed (http://snapzu.com/teamsnapzu), which has the pm box in the top right.
/t/lounge is also a tribe devoted to the exact purpose you describe.
As an additional note. You have to be following the user to see the "send pm" button.