In most cases, I assume the person who posts the lyrics knows how the song sounds, and the feeling they're trying to convey really is the feeling created by the full song, not just the lyrics. But since I'm usually not familiar with the songs from which lyrics are chosen, I have nothing but the words to go by. That's like someone who paints a beautiful picture telling you to paint the exact same thing, but they don't show you the picture. They just give you the colors of paint you'll need. Sure, colors can convey SOME meaning, but not merely as much as the spatial patterns and arrangements the artist creates with them.
Another thing that bugs me about this practice of lyric-posting is that it almost seems like a form of cheating... as if to say "I'm not artistic enough to write my own poems, but here's something someone else wrote that's close enough." Of course there's nothing wrong with not being artistic - I'm not very artistic myself - but when I read people's journals on here, I'm reading them to hear what THEY have to say. If everything is just quotes of other people's words, it seems kind of pointless to me.