Ignoring: shy or just not into you?

If a guy shows the initiative of liking you in the first place (by staring into your eyes, several times, and posting about you on FB) then when you try to contact him more often, if he tends to be tentative, is it because he is shy or he isn't really into me? P.S. We talked for a short time and then it stopped, but it wasn't like the candle was so strong it burnt out. Maybe I'm over reacting, but if a guy shows a strong interest in you in the first place, does that interest stay for a long time or die shortly?