When wouldn't you reply to a message someone sends you?

Let’s say someone sends you a facebook message, or a text message… when’s a time you wouldn’t reply to it?

I’m just wondering since, sometimes people don’t respond to what i write to them, such as “how’s your summer going” to a friend (not that close). What’s the deal with that?, do they not want to talk to me/ hang out?

I messaged someone that i secretly admire who is my friend, but he never replied to it :(.