I totally know what you mean.... hopefully they don't dissapear from here because they are back using... but many people just don't care about how their abscence creates worry to others.
You are obviously conscentious about other peoples feelings as well as your own... I am the same and hope not to dissappoint friends wether here on the internet or friends we see often, sometimes or rarely. You wrote a letter to a friend about having to avoid his invitation for some really valid reasons pertaining to your recovery. It was really proof of the caring kind of person you are. Your letter was written with great affection and your friend knew that you truly cared about his feelings.
I still receive no response from the friend I sometime post about here. It is so hard to accept how he hurts me by not responding to me. He even had been down right cruel before starting to ignore mee. Even though we are both sober he refuses to even give a word of support or acknowledgement that we are both going through the same thing. No one was closer to either of us than each other, for a year and a half. Funny we didn't fight over anything and when he was in rehab I sent emails of support because I ddn't want him to feel deserted and I had no intention of deserting him. People are sometimes heartless.... but you are a great example of a caring and decent person.