It could send the username to a remote logging place which timestamps every log. This way you can see the growth rate. If it can read the password it could also send out stats about the password (such as its length and if it passes various regexes to test it's strength).
You wouldn't need to build a logging thing, just use ccl.whiteacid.org, that will automatically also log the user agent, browser IP and referer and yes, it will timestamp each entry.
I don't know if logging the email would do much good.
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