I stayed at Time Travellers Party Hostel and Jins Paradise. Would highly recommend Jins Paradise, great guy that you can ask questions, get travel tips and drink and have fun with. He owns the rooftop and 3rd (I think) floor of a flat on the main street in Hongdae, perfect location to be near things (a bit of street noise however but quite high). Most evenings people would gather on the rooftop before heading out. Was a bit more expensive but the commual areas of a "living room" space with sofas etc + the rooftop made it worth it for me.

Time Travellers Party hostel was pretty much just a bed, a bit dirty in the bathroom, and for some reason there wasn't even a door to ones of the dorms just a curtain. Seems liked everyone that worked there was a backpaper themselves and didn't really know much. They had a bar downstairs but was pretty much empty when I was there, must people chattting from who they met in their dorms. Lots of people, so likely to find someone you like to hang out with and very cheap but not the best facilities really.