Well, there's a special type of being ignored that I, personally, am very familiar with.
It happened mostly back in my former high-school, but still does occasionally. When I was having a conversation with someone, which already is increasingly hard to maintain with time passing, and someone else came around, the person I was talking to simply turned and walked away with the just arrived one, ofttimes ignoring me, or interrupting me in the midst of speaking. Then there were cases of me having been asked to explain something and for that purpose to meet up somewhere at some specified time. The person supposed to come didn't come until an hour later, with someone else in company, telling me that other person had already explained it, and my help wasn't necessary any longer. And then, there are the classic cases of me being around people, and being blatantly ignored.
How I deal with it? I ignore it. If someone isn't interested in what I have to say, the person isn't worth my company. It's painful to be ignored, yes, it's saddening and upsetting, but the point is, if someone doesn't have that spark of decency and is incapable of basic respect, I am not interested in continuing the conversation anyway.
You have no explanation whatsoever for his apparent ignoring you. He is at liberty to leave and stop talking, and, furthermore, is not bound to explaining himself to you. The fact that he is only an online acquaintance substantiates that statement. Of course, it is 'unfriendly' and 'disrespectful' a deed of his, and you are free to feel hurt; he -should- have explained it, or made it clear otherwise.