Social media. I truly feel that it's been a disaster for just about everything. And yeah - that one could get us into political hot water really quickly. But basically, it's allowed people to "connect" without really being connected, to think they're being informed when they're being misled, and it's a huge time-sink.
My other pet peeve - shared by the inventor, btw, is single serving coffee pods. Colossal environmental mistake, which he really regrets.