Are you part of any other online embedded communities? These are the ones I spend some time on, mentioning RIOT everytime someone is looking for a HAL or RTOS to use.
Embedded.fm podcast slack (you need to be a patreon to enter):
There is a tag on stackoverflow. Never thought about searching on Reddit for embedded development. Thanks for sharing.
The /r/embedded subreddit looks to me as a good area for bringing RIOT-OS to the attention. A lot of people start out with Arduino and then need something more prowerful or more mature. I suggest RIOT-OS for projects about once a week over there.
There is also https://www.reddit.com/r/IOT/ which could be an interesting community for RIOT users