Why won't somebody reply back on Facebook?

[Andrew]

I've been speaking to this girl on Facebook and I asked her once that I'd like to meet up with you sometime and she replied with :-). Like today she put a status on Facebook saying that she didn't feel great so I messaged her asking if she is feeling any better, just trying to be nice and she reads it almost instantly but hasn't replied to it. The last thing she replied to me was the smiley face and that was on Friday, since then I've asked her how she's feeling and would she like to have a drink next week and she never replies to it but just shows as she's 'seen' it! Either way it would be nice to get some sort of reply as I think it's quite rude to just ignore it as it's a simple answer, yes or no!