Dave is correct in MOST of what he said. Except that this is a 3G carboy so that's not a 5G wort line
It's hard to tell from the pic but the wort is actually up just past the point where the carboy starts to get smaller (rounded). I generally keep it an inch or two below that mark to allow for enough headspace.
Airlocks are great if you have a LOT of headspace or the most active part of fermentation is over. For anything this full, it's a guaranteed blow off with an airlock because I fully expect the krausen to make it's way into the blowoff jar soon.
I had an airlock clog from the krausen once and it popped that sucker right off. I've heard of others having the carboy explode because the airlock got clogged and krausen dried in the neck sealing it so the bung wouldn't give. And yes, I use glass.
Since I'm working from home today I can perve on this a lot to keep tabs on activity. I just finished boiling some water that I'll cool and swap out soon so I can keep any yeast that get blown out. Too heavy of a concentration of Star San will kill them so I have to swap it out if I want to harvest the yeast. Then again, it's US-05 so it's cheap if I don't.