Well, there was one time (in regular email) that I had forgotten to answer someone. Guess for how long? For three years!!!! I had received to many emails for other stuff that her email was read but then pushed lower as other emails had come in... When I finally 'cleaned' my inbox, I suddenly came across an email that I hadn't remembered who this person was... OMG. I was so embarassed after I opened it and reread the email... I wrote back to her, profoundly apologising... She responded 'it's ok', but I never heard from her again... I deserved it. But I had never meant to 'forget' responding to her for so long...
If it happens to you on here, and you see they are online, could be that they read your message but after reading it, it doesn't show a number at top any longer, so I would suggest to try writing back to them. If they continue being online and not answering, then yeah,they are idiots just playing with your feelings. But if it's regular email, even fb, it could be that they meant well but just forgot to reply. Now you understand where I'm coming from. Maybe that's why I have so much patience... It was my only time that I waited so long. I have learned the hard way. Now I even message myself a notice on my phone to 'check messages from 'person' if i know I can't answer right away.