I've played Utopia for years. For those that don't know it, it's a strategy game where you have to build a strong and powerful province.
Players, may choose a role and race at the start and are then placed in a kingdom of 25 provinces. Together they strife to become the most powerful, biggest or most honorable kingdom on the island or even the entire world.

I'm not that familiar with other 'browser' games, but from what I have seen, Utopia may, with well over 100,000 players, be the most popular.
I think it is mostly due to the cooperative game play and of course the fact that you don't play against computers, but real people, with which you can talk to. Also, it is important to realize that graphics pay absolutely no role in this game. Everything is based on text. Your land, army, attacks, everything is displayed in numbers and text.

To answer your question whether it is possible to build something like that in delphi. Sure I think it is possible. Question is what do you want to recreate? (And I do not mean which type of game)