That 5 million number has come up recently in another context, though. FFXIV announced it had over 5 million "registered accounts".
No one is quite sure what SE means by registered accounts, though. Last year, SE said FFXIV had 2 million registered accounts and 500,000 daily logins. With 500,000 daily logins, that 2 million number is almost certainly subscriptions. So if "registered accounts" were what we would call subscriptions last year, then I think it's reasonable to believe that they are subscriptions this year as well.
As well, consider the latest trailer FFXIV has put out for the 3.1 patch. The last frame explicitly says, "Join Over 5 Million Adventurers Worldwide". This again cuts strongly in favor of FFXIV having 5 million current subscribers or equivalents.
Of course, the distribution of subscribers is probably different. WoW probably has more NA/EU players, while FFXIV has more Japanese players.
But in my view, FFXIV is now roughly the same size as WoW. With a new FFXIV content patch coming next week, and no new content in sight for WoW, it's possible that FFXIV will actually surpass WoW and claim the crown of "largest subscription MMO" in the near future.
It certainly puts a different light on Blizzard's decision to stop reporting subscription numbers. It's one thing to report losses when you're still at the top. It's another to confirm that you've slipped to second place.