People only tend to be receptive to unsolicited messaging over social media when it comes from an attractive person. Furthermore, it is considered gauche in some social contexts to message someone with whom you haven't established the requisite rapport.
I'm not a suave person by any means but I imagine that the recipients of your unsolicited messages might have been primarily confused if there was no distinguishable motive - perhaps you might have gotten a pass if it was concerning purely practical matters, such as the date for your next exam or something, but I generally do not message people unless they explicitly give me their contact details for a specific reason or message me first.