It's not surprising that YoBit are not tweeting about their issue, but it is strange for Poloniex! Are they both under DDoS attack or we are facing Armageddon here?
Althogh it was far from altcoin armageddon, exchanges have no excuse when they go offline. What kind of an online exchange allows that to happen? That's a red flag in my book. Albeit a minor one, but still a red flag. They should do better.