Look at, https://ethermine.org/statistics
There was a drop when the epoch hit. However it looks like hashrate is growing from somewhere. It topped at 65TH/s and now its back to 61TH/s so 6% total reduction, if you add to entire hashrate of ETH which is 280TH/s, it means 16.8TH/s has left the network. This obviously is just an estimate. Since its one pool and there are new GPUs going online everyday.
But that 16.8Th/s is almost 100K of E3's. However most likely there was some 4GB cards that stopped mining as well.