Thanks for sharing your observations. I’m seeing much the same, except that we’re using a 2 GB instance, so there’s less headroom. Also, thanks for pointing out that some measures of ‘free’ and ‘used’ memory are not necessarily helpful.
When I last rebooted the instance a few days ago, the initial memory use was 1.23 GB. Since then, it has gradually ramped up, and is now at 1.8 GB. The site remains reasonably responsive, for now.
The site doesn’t actually have many users, and there has been no recent increase in user registrations or activity. In the past month there have been about 20 new topics, about 100 posts, and about 4 daily engaged users.
I will continue to monitor things, and will post here if either the instance’s memory maxes out again, or the site becomes sluggish again, or both.