'Dance Moms': Hacked Again by 'Mean Girls'

"Dance Moms" cast members -- Abby Lee Miller, Mackenzie Ziegler and others -- got hacked Monday night in what looks like some brazen kids who keep coming back for more.

Sources close to the 'DM' cast members tell us the hackers have repeatedly gotten into their cellphones, iCloud, emails and PayPal accounts. The hackers have been posting phone numbers and other cast info on social media ... which is what happened again Monday night.

We're told Mackenzie awoke Tuesday to more than 2,000 messages.

The cast hired a P.I. 3 months ago to find the hacker, but they're super elusive. The P.I. believes the culprits are 4 underage girls, whom he's calling "The Mean Girls."