Why do they don't reply to the stuff I put on their Facebook wall?

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I have many friends that doesn't reply to the things I put on their wall. Those things aren't embarrassing, they are usually funny pictures about animals, or if I know that the person for example likes to play guitar (or anything else) I put cool/interesting stuff about guitar on their wall.
I know that they doesn't seeking pictures on the Internet, so it's not that they already seen it before.
I'm trying to be nice with that. I mean if I see those pictures they remind me about that 1 friend who might like it.
I don't do it too often, maybe like one time in 2-4 months.. I don't expect long-long conversations about the pictures, only at least a smiley or a like, so I know they have been seen it.

They are not shy about replying, so what's wrong? Should I stop sending them pictures?

They don't reply either if I send them links or pictures in PM.

Sorry for my English.
I moved to another country, so we don't meet so much, only in school holidays. If we don't talk on the internet, then we don't talk at all in that time during school.

If I ask them I bet I will got an answer like "sorry I didn't have time".. They said it when I ask them why didn't they reply to my messages (I waited for weeks). But every time I go to Facebook they are online...