If you are a member of mixi, go friend search 検索 2nd from the left on the upper menu and hit 検索 with no ristrictions, it returns ALL mixi members, which now ammount to 4.2 million+.
Update: Now the press release has been made. (via Fukumimi)
Update2: It is now 5,168,466 as of August 3, 2006.