If you lose data from an existing diagram lodge a support ticket. If the application isn't working at all, or a major part isn't working, you can post to the G+ community or lodge a support ticket. Data loss issues are always treated as a priority.
If the application does not work correctly for you, ensure you have tested with all browser plugins switched off. Plugins like disconnect block the Google and Dropbox scripts from loading by default. Also check on a second browser with plugins disabled, to ensure it's not a problem with the first browser and check the G+ news stream for issue reports before you submit an issue.
Also check the FAQ in case your problem is listed.
If you have a feature request, there is a feature request submission / voting system. If the request has already been submitted vote on it, otherwise, submit it yourself.
Belong on the G+ community. You are welcome to make general points here, or report minor functionality problems.