I feel like browser based MMOGs have been pretty steady for the last few years. Sure some games don’t even last for a year before shutting down their servers, but most of them are still around. The ones aimed at kids are probably have the biggest turnovers but still get lots of players (mainly because they’re always advertising for them on tv and online).
Also, I’ve seen very few that are purely browser based. I don’t like to have to download a new client just to try out a game that I may not play for more than two days. (Also the fact that my computer from college is too old to support the requirements for any current games so most of them don’t work for me anyway.)
Which country you live in also has a lot to do with what games are available. Korea has boat-loads of new online games each year, but they’re only accessible if you have a Korean SS#. There’s a chance those games will be picked up for international release eventually, but it’s usually like 4 or 5 years after the original came out.