Meh, some people (particularly in areas SL tends to serve) use their connections for e-mail only. Not me, but just sayin'
Then again, more and more people are using online e-mail, which isn't as tight of a system as POP3. Since they're pulling down the UI (and ads) with their e-mail, I'll bet the usage counts are a whole lot higher for those 1.5 million e-mails.
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