Nobody in their right mind wants to build a business that is 100% beholden to record labels. Facebook is focusing on building a platform first, and anything else would be a [time|money] distraction.

If their agreement is favourable enough for Facebook, perhaps they decided that the level of control that they had was good enough. With Spotify now building its business model around Facebook, Facebook's control is not far from outright ownership, it would seem. Facebook didn't need to spend x millions for any purchase.