[doublerabbit]

I found that Slashdot reposts from HN and Reddit reposts from Slashdot.

the cycle is continuous.

You do have imgur which at one point was largely teamed up with reddit and was originally built for reddit in the early days. However reddit got unhappy that they were making money off reddit’s user base so they quietly sank that ship.

Not sure where gfycat originated from but Digg was my first aggregator site before the makeover which I can only assume content came from HN too.