Like most, I have had people drift into, then out of, my life; The result of which has sometimes caused me concern, the feeling of loss, or conversely, the feeling of happiness and relief that they have moved away. It depends upon the person drifting away and the reasons for it largely I suppose.
I'm the type of person who tends to be able to draw a line beneath other people and move on fairly confidently but that's not because I don't value people, I do value them; I think it has more to do with the fact that I prefer to move forward, not backward, in life and if a person no longer wants to be a part of that, for whatever reason, then there's nothing I can really do but draw that line and move on with my own life.
I've often felt like people were not suitable, didn't match me or my ethos and beliefs and so have moved on and away from them and I understand that others do also. Life is time-limited after all and if one feels the pastures may be greener, more bountiful elsewhere, certainly from a relationship perspective, then I respect their decision to move on, as I hope they do mine...But they don't always do they?
Take the unfriending of a person on Facebook for instance...It often becomes a massive drama, often even more than two people drifting apart in the real world. I am not on facebook anymore, but I experienced this when I unfriended everyone prior to deleting my account.
The way I dealt with it was that I was moving on to greener pastures, emotionally in that case as Facebook was such a drain on my life and energy, and people could either join me, in the real world, or choose not to. Many chose not to and I'm ok with that.