comScore statistics indicate that Facebook drew even with MySpace in April. At that point, the networks interested about 115 million users each. If we're allowed mentally to extend the growth lines through today, Facebook's likely closer to 120 million, and MySpace has almost undoubtedly been left behind.
Keep an eye out for official global "nail in the coffin" reports, then. Since different research firms come up with varying figures, they'll trickle in for a while, making MySpace's position look quite bad.
Why the trend? Many have blamed it on the cluttered UI at MySpace. However, hope is on the way for Murdoch, as next week (Wednesday) MySpace is slated to roll out a new UI. We'll see if it reverses the trend.