I totally agree with Stan: engaging a community is not about the platform or the technology, but mostly about the regular work you will be doing to keep the engagement going.
However, there is a change in how people are engaging with social media platforms. As Mark Zuckerberg said in a public Q&A in November 2014:
“Messaging is one of the few things that people do more than social networking.”
Indeed the number of people using messaging apps is growing at a staggering pace as demonstrated in the following infographic:
So from these facts, I would suggest choosing a platform where members will be engaged mobile-first with a messaging-like experience. You could think about it as creating something like your own Whatsapp.
I believe so strongly in this idea that I built my own solution to manage a developer community of mine. As we discovered it can be used to help other community managers as well we made a white-label private messaging app from it.