All:
I would have to agree (at least in part) with Sweetnighter here, in the sense that the "definition" of "empire" may have changed over the years.
As Sweetnighter notes, an "empire" used to be, at least primarily, the ownership and control of "real estate" (and economies of those countries it held), usually by force or threat of force. However, I think the operative word is no longer "real estate" but "control" - in any sense, by any means.
In that regard, it is not simply money that determines the "American" empire. Consider:
-Culture. American "culture" has become globally predominant. American "exports" re music, fashion, film, etc. have spread to - and, in a figurative if not literal sense, predominate in - almost every country. Think of Coca-Cola, McDonald's (in all but two of the sovereign nations of the world), Disney, etc. Yes, there are other centers for fashion (Milan, London, Paris et al), music (England, Germany, Italy et al), film (India, Italy, England et al), etc., and other "global" "brands" (e.g., Virgin). But America is without question the "cultural leader" of the world. This creates a sort of "hegemony" all by itself. As an aside, the U.S. TV media "stole" most of its "reality TV" ideas from British programs. Yet the British were unable to "sell" them to the rest of the world, while the U.S. has been successfully selling them throughout the globe (and thereby continuing to "dumb down" the cultures of other countries...).
-Language. More people speak English than any other language (except perhaps Chinese, but that is simply due to sheer numbers). True, the British started this during the creation of their empire. But the U.S. influence in "forcing" English on almost every other nation cannot be questioned. Somehow, Americans don't think they should have to learn to speak a second language; yet they believe everyone in the whole world should speak theirs. And more people in more countries speak English than any other language other than their native tongue (i.e., as a second language). If this is not a global "hegemony" of sorts - as well as one definition of "empire" - I don't know what is.
-Money. Whether up or down, the dollar still determines the remainder of the world's financial markets. And the U.S. economy, even at its worst, is still bigger - by far - than the economy of any other country, or even group of countries. Indeed, the creation of a "United States of Europe" - and the development of the euro, etc. - is a direct reflection of just how powerful the American economy really is: i.e., that it takes over a dozen major countries joining together to simply create a relative "parity" of economies.
This is just a very quick, simplistic look at the global scene. I know there are many more examples of America's "empire," as "newly" defined. However, I am in a hurry. If I think of more, I will post them.
Peace.