With regard of the software, I have no reasons to prefer pump.io.
(No deep knowledge of any social network in particular).
It's just that, because of the Identi.ca migration, it happened that I began curious to tinker with it instead of with Diaspora or Friendica.
It's the same thing as if you need to edit video and you know that a friend uses Kdenlive, you give it a try. If it happens that your friend uses Openshot, you would begin to use Openshot instead of Kdenlive. And after that, you just keep on using the one that you installed (lazynes...).
In social networks, in addition to that, you create links with people, so if it's enjoying, there's little motivation to go to other place or try a different software.
I know it's not fair for fantastic projects as Friendica and Diaspora but.. what can I say... these things happen.