How to know if someone is genuinely interested in u?

When someone shows so much interest, glance, smile and all I received to be real after watching them for some time. But when I finally followed them on social media they wouldn’t do anything, they wouldn’t even react to one of my posts or stories. Does that mean I wrongly read the earlier signals or they just wanted to play a game with me? Why do guys show so much interest if they can’t go for that person which only creates confusion. I seriously want to know the reason why do guys do that? I’m so confused and disappointed right now