Until 8 p.m. Until 8 p.m. Until 7 p.m. A quick look at the chart shows that just because brokers use the same platform to trade after hours, it doesn't mean that they offer the same hours. All four brokers that offer after-hours trading via Redibook offer different features.
Volume has an awful lot to do with their success.
Instinet is the after-hours leader, averaging 66 million shares traded in both premarket and postmarket trading during March. Island averaged 18 million shares a day over that span. These robust volumes make trades easier to execute. With more buy bids and sell bids on a particular ECN's order books, you have a greater chance of finding a buyer or seller at your desired price. Even now, however, there is more liquidity than there was a year ago.