MLS has become more and more profitable on the club level, mainly because clubs have started realizing the importance of soccer specific stadiums. When clubs were renting out venues that a) were much too big and b) were very very costly (some clubs spending between $50-100K per match), this already put them in a disadvantage. Not to mention that they were also losing out on other very important revenues (mostly concessions and parking) which were all being collected by the stadium owners (and NFL clubs that ran these stadiums).
Since the onset of clubs building soccer specific stadiums, being able to keep all revenue streams, including additional events such as concerts, the balance sheets have been much more favorable. In the last few years, clubs such as LA Galaxy, Seattle, Philadelphia, Portland, and Toronto that have been building impressive attendance figures for MLS, which has also helped in this regard.
Over the period of 2011 to 2015 the Average Projected MLS Stadium Revenues and Expenditures will be $15M in total revenues and $9.5M in expenditures. Although these aren't Man U or Barcelona type of numbers, but profits non-the-less. As of today - there are only several clubs that do not have their own soccer specific stadium (and I believe those that don't are planning or even currently building). Plus, any new expansion franchise group MUST have finalized plans of a stadium for the league to even consider them as a potential franchise candidate.
Unlike the other "major" leagues in the US, MLS was founded in 1993, with its first matches kicking off in 1996, so considering that this league is still in its "infancy" it has shown respectable growth both in the front office, in the stands, and on the pitch. It has surpassed NHL in average attendance, and is slowly catching NBA.
I truly feel that when MLS has the luxury of being in around for 30-40 years, which would still make it much younger than most football clubs around the world, as well as many franchises in other sports in the US, it will be even more impressive.