SO for starters, asking people that use Slack or Teams why they dislike Yammer would be one sided as chances are, they prefer Slack and Teams to Yammer, hence them using it. Also, Facebook and Twitter (I got most of my responses from Facebook as I have friends that work in the same space that i do) provided me with a broader base to work from.
As for reach, expanding on Facebook and Twitter, anyone who shares or comments on my post, would expose that in their feed to their followers/friends and so its compounded to followers of followers and friends of friends so the reach is rather substantial.
If i asked the question on Yammer, i would navigate to my specific network and ask it there, and then i would have to copy/paste the same question to the other networks i belong to.
Discovery is great if you have a few thousand people in the same “general” network. However, discovery is limited to the people that are part of the network. Discovery works great across groups though, which is really cool.
External Networks are awesome too.