My conclusions are similar. My follower/following ratio has been stable at about 90/100 for a while now. Those that lose quality in my eyes or are no longer relevant to me (because I do not use their product anymore, etc.) I unfollow rigorously. Signal to noise ratio is one of the best out there IMHO, even with annoying promotions. I would really welcome any suggestions for something better. Only small thing I would improve is enabling people to answer on pms, even if they are not followed by the account that pmed them.