Competition is usually a good thing for consumers. And it probably is in this case, except right now I have the choice in making an account on smugmug where one prosumer photographer friend puts his pictures, 500px where another semi-pro photographer friend hangs out, or of flickr where several other friends who mostly have compact cameras are. That's not ideal.
While it's possible that Flickr could benefit from the existence of smugmug and 500px, IMHO it's more likely that one or more of this group of competitors will eventually be forced out of business, especially as the grandparent post makes the point that due to facebook, instagram etc, these sites are now catering to a niche. Of the three (flickr, 500px, smugmug) only flickr has the scent of failure on it. I hope they shake it off, but which one would you pick as a winner?
And in that space smugmug and 500px are also aiming at the DSLR owners who are willing to pay double-digit $ per year.