According to the Puget Sound Business Journal, the Seattle Sounders have already sold 20,000 season tickets for their inaugural season. Assuming sales in other cities remain consistent, this means that MLS should have three teams average more than 20,000 fans per game next year (Los Angeles and Toronto both accomplished the feat the last two years running.)
To put this into perspective, the Seattle Mariners have only sold 14,000 season tickets for next year. I realize that baseball season tickets are much more expensive (due to the large number of games) but it's still impressive that the Sounders are likely to regularly sell out the 24,500 seat soccer capacity for Qwest Field next year.
To add even more perspective, the USL1 version of the Sounders averaged a touch under 3,400 fans per game over the last two years. It just goes to show that USL1 attendance can be pretty much ignored when sizing up potential MLS markets.