What usually happens - and if you're a FB user I'm quietly confident that you will have seen something similar - is that the post refers to 'an angel in heaven' or 'a star in heaven', or some other similar silliness. There's a couple of things about this kind of post which metaphorically twist my nuts (try not to have a visual about that - just leave it to me).
Firstly, it's a planned manipulation of people's emotions - the overwhelming majority of adults will of course have a relative who has died (recently or otherwise) and whose presence is sorely missed. That kind of thing tends to be a fact of life for almost everyone - definitely not a unique experience. Posts such as this, however, quietly imply that such emotional suffering is indeed unusual, if not unique (humans, locked within a single mind, tend to read these things alone, and experience the words in the first person). These posts tug at heartstrings (if we have strings in our hearts, which, if I remember my year six biology lessons correctly, we don't) and fire emotional thoughts in order to engage us. Sorrow and sadness are powerful engagement - not to mention, manipulation - tools. Whatever the motivation is for posts such as these, my suspicion is that they simply spread sadness, and I don't believe that is a good thing, no matter what the reason for it may be.
Secondly, I have yet to see a post which asks me about a missed person who is in hell - or, more realistically, a loved one who is simply gone, no longer alive, and has therefore ceased to exist (I'm resisting the Dead Parrot Sketch references). Apparently - or so it would seem the assumption must be - all our loved ones go to heaven, in much the same way as TV show pyschics only ever 'speak' to the dead who are in heaven. Nobody is ever anywhere else...always heaven. I've never seen a psychic tell a client, for example: "She says it's bloody hot, agonizing, and there's no end to her suffering!" That's because it's an industry which panders to a person's needs to feel better about losing someone from their life. I have a feeling that these online posts constitute a subtle support structure for this kind of industry.
Since a great many people believe in some kind of afterlife - and most of them seem to believe in the concept of a heaven or hell scenario - isn't it just a little bit curious that everyone we have loved (with perhaps the odd exception, such as Uncle Billy the Murderer) is assumed to have gone to heaven?
I'd love to see this belief debated with, perhaps, a Catholic priest, for whom the concept of hell and eternal suffering is a major motivational and doctrinal force. If hell exists in the minds of believers, who is going there? If we polled believers in the concept, I strongly suspect that we would find that a huge majority of folks would say that their loved ones are definitely in heaven. If they were right, the ramifications for Abrahamic religious teachings would be huge, since it would indicate that getting into hell is a lot more difficult than we may think...
For me, the truth is much more simple. Nobody goes to heaven or hell - up, down, or anywhere. We die, and the lights and the sounds and the touches and tastes and the smells end, much as they do when we fall asleep, only without thoughst and dreams, too. It ends, and our time is gone. the finality makes sense to me - whereas the conflicting and competing nonsensical ramblings of the world's religions simply serve to conceal from our emotional, prone-to-panic minds that the end is final.
These Facebook posts are equally nonsensical in their conception. They prey upon superstition and sorrow and vain hope, and for that I despise them and their negativity disguised as empathy. If we take a step back and ask ourselves - especially if we have a religious belief - why we all assume that our missed person is in heaven (perhaps despite the fact that heaven is supposed to be a difficult place to get into), it may offer some perspective upon those beliefs, which I suspect we hold simply in order to feel better about the truth - the truth about death which we find so hard to face.
Yes, of course, I have feelings of sadness for people who are no longer alive. I have feelings of longing, and I wish that I could once again experience their presence in my life, however passing around what amounts to an emotional chain letter seems to be a very inappropriate and inadequate way to honour their memories...