An extremely detailed summary of 2015's attendance numbers is
here.
Cliff notes:
Average attendance was 21,550, an increase over just over 2,000 from last year
Median attendance was 20,018, an increase of just less than 1,500 from last year
Both are records and are numbers that were unimaginable five years ago. I remember when everything over 15,000 was deemed to be pretty great. This year Colorado averaged 15,657, the worst in the league.
The big jump in 2015 numbers were powered by the addition of NYCFC and Orlando, the subtraction of Chivas USA, and San Jose's new stadium. New England (17.7%) and LA (10%) also saw double-digit increases.
There were also a number of minor decreases around the league. The most widespread drag on attendance is that the new national TV deal pushed more games out of Saturday night, which is the best time slot for crowds. The league seems to think that trading some better-attended Saturday night games for long-term regular TV slots on Friday and Sunday nights is worth it. I agree.
With no structural changes coming next year (popular expansion teams, Chivas USA purging, new stadia) and with several teams at or near capacity already, it's very unlikely that 2016 will see nearly the growth that happened in 2015. In fact, it's possible we'll see a decline in 2016 with Orlando moving into a smaller stadium at some point during the year, but the league is in a good place nonetheless. Atlanta looks like it has strong potential to be an attendance juggernaut in 2017, perhaps even Seattle-sized. Orlando-sized at worst.
The bottom five clubs are all legacy teams founded in the 1990s - Chicago, Colorado, Columbus, Dallas, DC. I expect DC to perk up significantly when their new stadium opens in 2018 (ish), both because RFK is a total dump and because DC will be able to afford to invest more into the club on and off the field. They may be the only team in the league who can legitimately cry poor at this point in time. Chicago and Colorado have terrible ownership/management and have sucked on the field for a while as well.
It's kind of crazy to think that the Portland Timbers were a slight net drag on average attendance all season because of Providence Park's capacity, although they boosted the median.