Mountain biking is clearly on a downturn in the US, and has been for a couple of years, at least as far as NORBA membership is concerned. Also, there are 1/2 the number of MTB races than there were 5 years ago, and fewer races.
Some of this can be attributed to the "Lance" factor - you know, the yellow jersey wearer. He made riding skinny tires fashionable. . . again. Some of the decline is from NORBA mismanagement of the racing scene. Also, there hasn't been a dominant, or even competitive american male MTB racer in years in the XC arena. And some is from the changing nature of fickle human beings.
There may be growth in certain regional areas where there's a strong core and good race promoters. . . BC comes to mind.
Mountain biking will always be around and it may even turn around and start to grow again in the coming years.