Per the Gatherer rulings:
The sacrifice occurs only if you cast it using its own ability. If you cast it using some other effect (for instance, if it gained flash from Vedalken Orrery), then it won’t be sacrificed. (2009-10-01)
As a note, if you cast Necromancy as normal without using its first ability to cast it at instant speed, you also wouldn't have to sacrifice it.