Is the network hash rate on bitcoincharts.com is accurate or statistically estimated? It's fluctuated from 7Thps to 4.7Thps, which is a huge drop.
Why would 33% of the network switch off mining?
The hashrate is calculated from rates of production during the current difficulty. Immediately after a difficulty change there isn't enough historical data from the new difficulty to give a good indication of the network's total hashrate, and so the rate that gets displayed is off. That's what was happening when you saw it at 7THps.
There is plenty of historical information, it's just that BitcoinCharts uses the easy and stupid method for calculating the hash rate which doesn't work properly while shortly after a difficulty change. It is very easy to take the difficulty change into account; I don't know why BitcoinCharts still hasn't fixed it.