You know if your 'randomfriend' dont reply to your tells, can just assumed that they're either:

1. Busy and dont feel like answering.
2. AFK.
3. Don't give a f* about your tell and just ignoring you.
4. Wants to be left alone.

Best course of action: if they're a friend, just leave them be. If you were to meet them for something, wait a few min and send a /tell back. If no answer, then just leave it be.

Is it absolutely necessary to be able to keep track of everyone's every single waking (or non-waking) moment (as in non-AFK)? We know back in the old days, we call our friends at a home phone and if no one answer we'll just leave a message. Call back later if it's important if not, just wait to get a call back (or none back and that's a sign that either they're in trouble or they dont wanted to be bother with).