Devs, don't you plan to make your own slack channel? A lot of people would be interested in using it. Also it can increase your score at different ICO trackers.
I don't know if a slack channel will increase your score on ICO trackers, but Emiba has a Twitter account, a Telegram Channel and many more. For a slack channel, you will need additional team members, which is in my opinion not necessary.
I guess few social networks are sufficient for marketing purpose. Because max channel can more difficulty to track members and for post new updates.