Although most MLS teams lose money, the league is strong financially becuase is has ownership groups that are commited to the league's success - and are extrememly, extrememly rich. Billionaire Phil Anschutz started the league - he was also the pioneer of cable television and fiber optics - and he has since sold the majority of the teams to individual owners - most of whom are billionaires themselves.
The league added Toronto FC this year and will expand to San Jose next year to bring the league to 14 teams. The league plans to top out at around 20 teams, which is the number playing in most major leagues throughout the world.
No teams are folding at present. The last time the MLS lost teams was when the Tampa Bay Mutiny and the Miami Fusion folded back in the early '90s. Since then the league has undergone numerous changes including more lucrative television contract, better success merchandising and of course - signing international stars like Beckham, Angel, Xavier, Blanco, etc.
The main reason I think you'll see soccer continue to grow though, is because of the commitment of the owners to make it work. These are people who know how to make money and recognize potential. You don't get to be a billionaire by making stupid decisions and I believe the MLS will grow into a respectable soccer league.