I admit, I have been in exactly the OP's position. Thinking of someone, send them a nice message (sometimes FB, sometimes email), then... nothing. And yeah, it irritates me, and I start to get a little paranoid and wonder maybe they really don't like me that much, or did I say something wrong... But I feel like I have to let it go, or else I would be poisoned with irritation and anxiety, you know? With one message there are a lot of things that could lead it astray or make it look like it's been read when it hasn't, or the person could just be busy without trying to send me any particular negative message.
I would give it a week. Then, I might send the person another message, perhaps through another medium (email instead of FB, for example). I wouldn't ask if they saw my first message, but I might reference it like, "The project I mentioned working on last week is finally finished! It was stressful but ultimately came out well..." I would give them another update on me, and maybe ask more specific questions about them--like if in the first message I said, "How's the family?" maybe this time I would say, "So how's Bob doing? Has he settled into his 'new' job? How is Susie doing in college--what's her major again?"
If they never saw the first message, they might realize they missed something and go looking for it. If they saw the first message but just hadn't gotten around to replying, they will probably do so now, realizing you are truly interested. If they saw the first message and are deliberately ignoring it, they will either continue to ignore the new message, or might send a perfunctory reply. After two messages being unanswered (especially if I used two different media) I probably wouldn't try again for a while, and might start to reassess the friendship in my mind (if this is the only contact we have).