Going forward, we won’t allow accounts, images, or hashtags dedicated to glorifying, promoting, or encouraging self-harm...
[A]ny account found encouraging or urging users to embrace anorexia, bulimia, or other eating disorders; or to cut, harm themselves, or commit suicide will result in a disabled account without warning...hashtags that actively promote self-harm, such as “thinspiration,” “probulimia,” and “proanorexia,” are no longer searchable.
They were careful to scope this out:
It is important to note that this guideline does not extend to accounts created to constructively discuss, or document personal experiences that show any form of self-harm where the intention is recovery or open discussion.
From the fearsome media perspective, we can view Instagram's move as an attempt to prevent accruing a reputation for supporting such often-scary materials. Pinterest took similar steps last month.
(thanks to Steven Kaye)