Are they? For sure they have the largest market share, but isn't lack of substitute goods a necessary condition for monopoly? IANAL, but I use Bing all the time on IE9 because it's the default search engine and most of the time I can't even tell the difference. I certainly haven't found a reason to change it.

It all depends on how you define Twitter's (and Google's) market, doesn't it? I'm pretty sure the gap in marketshare between Twitter and closely similar services like Identi.ca, Plurk or Yammer is actually larger than the gap between Google and Binghoo.

theres plenty of competition. So now we're supporting the government stepping in, when theres plenty of competition but just nothing thats good enough to compete. This is not an America I want to live in.

Are you a lawyer? as I understand it a court would have to declare a monopoly and in this case none has.

Purely from a commonsense point of view however there is no such thing as a monopoly on information retrieval.