Total attendance due to number of games played yes. But not average attendance. Last year, the NHL averaged 17,502 fans per game, the NBA 17,826 and MLS 19,148 (source: memory, OK- it looks like 2011 was the first time MLS surpassed the NBA and NHL in average attendance -
http://www.businessinsider.com/chart-mls-surpasses-nba-and-nhl-in-attendance-2011-11 and it has been ahead in that metric every year since I believe). So far this year the average attendance is up to 20,979 (in large part thanks to Orlando). This is important because more than any other sport, soccer is better live. So far this year the high average is Seattle at 40,236 (they play in Century Link, no soccer specific stadium necessary when you continually draw like that), low average is 14,458 (DC United), and the median is 20,048 and 20,025 (20 teams so I just looked at the average attendance of the 10th (Real Salt Lake) and 11th team (Sporting Kansas City) - here I looked up MLS 2015 season on wikipedia and some weirdo put together an attendance table). MLS does have a built in advantage in that Seattle and Orlando play in football stadiums and NYCFC in Yankee stadium (this summer has to suck for the ground crew). There are 3 MLS teams that have largish soccer specific stadia (Toronto, LA Galaxy, and NYRB) in the 27-30,000 range. Then there are around a half dozen teams that play in stadia near the 20K mark that are at 95-100% capacity and a half dozen teams that play in stadia near the 20K mark that are 70-80% capacity (the 14-18K averages).
TV viewership is far lower than the other sports if you just look at MLS. I think it is averaging around 240K viewers per game on ESPN2, FS1, and Unimas this year. You can watch all the games live if you buy the MLS streaming package and they've seen a considerable uptick in these sales the last couple of years. I'm sure sales aren't what say MLB streaming packages are, but the league makes good money from these. Fox, ESPN, and Unimas did pay a chunk of change for MLS rights over the next 8 years. The hope is that regular time slots (Friday night - Unimas, Sunday night - ESPN2 and FS1) will help. The league has never really had regular time slots before. They really hurt their TV rating average this year by showing a couple of midweek, Wednesday night games with little or no advertising. I believe 7 people watched these.
If we are just talking about the popularity of the sport, the men's and women's national teams are both huge draws in big tournaments. People also tune into the EPL on NBC and the NBC family of stations Saturday morning, Sunday morning, and Monday afternoon the rest of the year. Also, MLS has a competitor. NASL is not the official second tier in the U.S. USL Pro is. NASL (Cosmos, Tampa Bay Rowdies, Ft. Lauderdale Strikers, Edmonton, etc.) is trying to compete with MLS. Most think the minimum salary increase, new TV package, and the partial knee capping of the league by stealing Minnesota in a couple of seasons will eventually spell the end of the NASL, but our local team the Indy 11 still sells out 12K or so a game and has their own local TV coverage. I should note, with the exception of the Cosmos and Minnesota, the other NASL teams don't typically draw this well.
Anyway, MLS is stable and doesn't show any sign of shrinking like the NASL did (it never went away, but it did whither to almost non-existence). The true concern of many is that it seems impossible to have a stable woman's league that doesn't lose money.
I'll see if I can find it but I read a trenchant article a few years ago that made the point that after the NFL, everything is essentially a niche sport. It started out by pointing out that NFL preseason games outdrew the World Series. For the record, I'm not claiming soccer is not an unstoppable juggernaut. It does have enough of a following to survive. It doesn't really matter to me where it ranks relative to the other sports so long as I can attend games and watch games on TV from time to time.
For the record, if someone put a gun against my head I could name every MLS, NHL, NBA, NFL, and MLB team if you gave me a few minutes to gather myself and get used have pants full of tinkle water. Also,
@Steel City Magic mentioned the Pittsburgh River Hounds which floored me. What's next, the Carolina Railhawks or Rochester Rhinos?
Sorry for the tangent.