Until everyone uses the same ECN, your potential number of trading partners is obviously limited. In the meantime, make certain that you work with a broker that has access to quotes on other ECNs and that you can have your order routed to another ECN if it has a better price. During the regular trading day, buyers and sellers of most stocks can readily trade with one another. After hours, there's generally much lower trading volume and it may take much longer to find buyers and sellers. In fact, some stocks may not trade at all during extended hours.
Less liquidity translates into more price volatility.
Less trading volume also means wider spreads between the bid and ask. Therefore it's possible you may not get as favorable a price as during regular market hours. The best markets for trading are those with the biggest number of participants, the ones where you're likely to find someone who wants to sell what you want to buy or buy what you want to sell. So think carefully before trading in the wee hours of the night. It might be wiser to wait until morning to make a decision, to see where your stock opens and if there's been any more news about the company.