I'll take a shot at starting a conversation or saying hello but where it goes from there is up to you. Looking back at all of my messages online (not just on EP) everything depends on the other person. It may start out as small talk then develop into a deeper conversation. As soon as the conversation goes back to small talk, that's it. Long messages or responses get long replies from me. Short messages get short replies. Most of the time this is true. Even in face-to-face conversations. If you want to talk, I'll listen. If you want to discuss, I'll participate. It's like I wait to see what you're going to do or say before I make any moves. Or it could be trust. I don't know why, but yes, where the conversation goes is totally up to you.
** This generally applies to someone I've just met. Once I've known you for a while, I don't mind talking or taking the lead. Maybe it's shyness or something bigger.