I used to get that feeling now and then, not a need to "investigate" them, but a small sense of lonliness seeing that a place that was bustling with conversation and community isn't anymore.
I think it lessens as you age though.
There are countless situations in life that can cause people to move on from a group, hobby, social circle, forum, etc. I'd argue that most of them are perfectly normal and healthy, like having kids, moving, finding new hobbies, changing jobs, life events changing free time, making new friends, finding a new partner, etc.
Sometimes people find their way back, sometimes they don't.
People don't stay the same forever, so their interests and how they spend their time won't either. I think life would be very boring if that wasn't the case.