It seems that whatever its rivals do to win market share from it nothing can dent the monolith that is CommSec. The latest online broking report, Investment Trends, shows that CommSec has increased its market share over the past six months, while nabtrade and Westpac Online Investing, which have been much more active in upgrading and promoting their sites, have stood still or gone backwards.According to the Investment Trends report, CommSec's share increased by two percentage points, to 51 per cent, during the first half of the year (the survey asked investors to identify their primary online broker relationship).During the same period, E*Trade's share went up one percentage point, to 17 per cent; Westpac's fell one percentage point, to seven per cent; and nabtrade's was steady at five per cent.Last year, NAB re-branded and re-launched its service, with the aim of grabbing a much bigger slice of the online brokerage market. Nabtrade has innovative technology and has already undergone a couple of upgrades.Investment Trends' senior analyst, Pawel Rokicki, said it was too early to judge the success or otherwise of nabtrade, which is still in transition from its old online trading platform to a new one.Investment Trends' data shows that nabtrade is picking up 19 per cent of online investors who are new to market.However, the Investment Trends' data shows that the big banks are fighting over a market that is getting smaller. Over the past year, the number of online share traders in Australia has fallen from 615,000 to 530,000 (people who placed at least one share trade with an online broker during the period).A curious finding is that the online brokers with the highest customer satisfaction ratings have tiny market share figures. CMC Markets Stockbroking, which has the highest satisfaction score, has a four per cent share. And Bell Direct, which is ranked number two for satisfaction, has a similar share.