You see the person out at a bar and they don’t make a point to speak to you or a restaurant and they don’t at least say hello to you.
You send the person an invitation and they don’t accept it or let you know they can’t make it or give you a reason why they didn’t come after the event.
You call the person and they don’t answer; you leave a voicemail and they don’t call back.
You text the person and they don’t text back. "K" is not an acceptable response unless it's followed up with greater context/detail within 12 hours or the person is abroad.
You tweet at the person and they don’t respond. (Follow back is not required if they RT or reply to you.)
You write on the person’s Facebook wall or message the person but they don’t respond; you send a friend request but they don’t accept.
You like a photo on Instagram and follow them; but they never follow you back or like any of your photos (follow back is not required if they "like" any of your photos).
You send the person a LinkedIn connection request but they never respond; you send a recommendation request and they never respond.