Which brings the point, who would ever sign on to use Slack/continue to use Slack when Bezos & Co. could be scouring through your sensitive company conversations?
If somebody can prove me wrong, however, that would be lovely :)
Regardless though, expecting Amazon to refrain from taking the proprietary information they have access to (in any form) and drilling you into the ground with their own twist on it seems like a bad bet, just ask Nucleus.
Companies that don't want to entrust their sensitive conversations to Slack, etc., opt to use on-site Hipchat servers, or other means of communication. Amazon buying Slack doesn't significantly alter their calculus.