Complaining that your G+ engagement has gone down because you're refusing to use Collections, is like being on Twitter 8+ years ago and complaining that your engagement has gone down because you refused to use hashtags.

I've been on G+ since Beta and I've seen this cycle repeat itself over and over: G+ releases new feature meant to increase engagement. Some folks embrace the new feature and their engagement levels go up. Other folks whine, dig in their heels, refuse to use the new feature, and their engagement goes down and they blame the network. Which is rather like not updating your website past its 1998 form and being surprised that it stops ranking well in Google.

Use Collections or don't, but it has been made quite clear that they are a heavily emphasized feature and thus you can't be surprised when refusing to use them doesn't benefit you in the end.
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