Yes, they pay. Because it helps keeping users active, in case of Instagram, for example. And in slack's case, I think when you install the app you can have it f

Yes, they pay. Because it helps keeping users active, in case of Instagram, for example. And in slack's case, I think when you install the app you can have it for free (with limitations), or pay to remove said limitations. In that case, slack may not (IMO) pay then.