Y'all may also be experiencing this that random people follow you and well when you follow them back, they don't even say hi.I think that there can be a number of reasons behind this.Either they are a pedo or they want you to start a convo coz they are shy or it could be a new kind of OCD(Obsessive Compulsive Disorder in which people do weird stuff and if they don't then that causes them to feel anxious,uneasy etc etc.But I wanna know that what do you do when you are stuck into this kind of situation.I know some people don't follow back coz that saves them time or they must have their own reasons but I can't do that coz Personally I think that it's rude not to follow back or reply and obviously what goes around comes around.
I follow them back judging by their status message and wall posts If they are perv-y I don't follow back
I usually follow them back thinking they a have good reason doing it but to initiate a talk I wait for them .. -and when they keep their silence I let it go...(applies to new and have not talk to the person at all)
I'll just be like "I'll wait till they start convo" if they never start one.. Well I totally forgot them