There are lots of ways they could use Twitter to make money. They could put up ads, as Facebook has, or they could charge all media companies a fee to use the service, as several blogging platforms do (in return for more robust SDK that lets them integrate it in interesting ways).
Making money should be Twitter's last problem.
I think the bigger problem is that the permanence of its user base is highly speculative at this point. A lot of people have gotten in on the fad lately, but there isn't much there to point to it being anything bigger than that ... a fad. It's not a good idea to build a profit model around a fickle user base.
While I'm no Facebook devotee, at least it offers a proven set of functions, and only really stands to grow its user base over time, barring some major unforeseen fuck-up by the developers/marketers.
Anyway, Facebook is typically for keeping in touch with those people you already know, or new people you meet on any given day. "Look me up on Facebook" is a far more common use than is meeting people through the service itself.