Unfortunately, while I prefer apps like EyeEm (and I'm curious to check out Steller), the real reason that Instagram works for marketing is that it's also used by people who aren't photographers. So while you might be building a decent following on flickr or 500px, it's other photographers, who probably won't be looking to hire you to shoot their campaign/wedding. If on the other hand you're looking to make a name for yourself doing workshops/training for other photographers then maybe they're a perfect place to market yourself.
I don’t expect to move over to either of these as my main social network, but Twitter’s latest waves of jack-assery has kickstarted the conversation again. There are a few problems with these other services, but the main thing is that they just haven’t managed to get enough people to stick around and use them for long. People tend to try them and bounce off them pretty quickly. I know I have bounced off each of them numerous times.
500px has long been popular with the photography community with it's clean approach to photo sharing. No hashtags muddle this pond, 500px is all about sharing great work. Curated collections are excellent and regularly updated, whilst the Exif data upload is a nice touch to delve into the technical workings in-camera (Flickr also has this function). There are plenty of similarities with Flickr in terms of it being a platform angled towards promotion of the best creative work rather than popular accounts and sponsored posts.
A simple chat (slack style) it's enough. Just having the possibility to send messages either single user or team (#channel). But please don't forgive to give an useful appeal to mobile app (worst error of competitors is, have a good app but totally no mobile friendly!) i would expect to have a similar whatsapp or slack app for messaging within the slack mobile app!