Back in the days, was easy to create a platform and have engagement - BUT TODAY - there are hundreds of social platforms but none of them work - and the reason is that technology is evolving - everything is evolving around us - so this means that a Marketer needs to think twice before creating a platform.

At this time, platforms that only talk (WebhostingTalk,stackexchange) will decrease in engagement and majority of users - will go to Platforms that have an application+talk.

Example:
WebHostingTalk - here users only talk and slowly - this page is dying. The same content you find on on WHT - you can find everywhere else - so looking from the ROI prespective, a platform such WHT will not work out.

On the other side, GNS3.com - here is platform based on user activity + application. From this platform, i have two types of engagement (Talk + use my software). By using my software, users will start to give questions, faqs, answers - so this way your platform community will grow.
Examples of this kind of platforms are:
Gns3.com
Spiceworks.com

So in conclusion - for a platform to have engagement,activity,actions - you have to provide them 2+ of engagement. (Talk, download your application). Combining this two - you will have a strong community platform.
Take example from GNS3.com - they are a great example for anyone who want to create a community platform.