twitter is internet plumbing. it doesn't matter if it makes money, it drives ridiculous amounts of ancillary commerce. There's plenty of people that are willing to pay money to keep it alive because it serves their business. Infrastructure tends to be money losing businesses when they start; think youtube and more recently, facebook.
Most physical infrastructure is insanely costly to create in the first place. If a utility charges too much money for your liking and you don't have a couple billion to compete with them, tough shit.
The infrastructure of Facebook and YouTube and Twitter could be replicated by a small team of smart developers in a matter of months, probably costing only a few million dollars. The only thing they have going for them is network effects, which are less powerful than we think when there is a viable competitor (e.g. MySpace being overtaken by Facebook).
