We have tools to show this in our moderator section but as we're not allowed to show you any of the detail in there (of which this probably counts) I built you a query on data.stackexchange.com:

 - http://data.stackexchange.com/ubuntu/query/80302/new-users-per-day

Those are the "new user" counts for each day. If you want a running total, you'll need to build something that accumulates a running total (fairly simple, either in a script or a spreadsheet).

Note: the data on that site isn't live. The latest data on there at the moment is from the end of June. They do dumps every few months.

For other site stats in less depth (but more current) you can look at Stack Exchange:

 - http://stackexchange.com/sites#traffic


##Edit: Got bored; made a graph.

![enter image description here][2]

You can download the spreadsheet from:

 - http://people.ubuntu.com/~oli/au-users-20120626.ods


  [1]: http://askubuntu.com/users
  [2]: https://i.sstatic.net/Ka2Lk.png