The “videos” on YouTube are web pages. But they are very object-y, with the video player being the object people see, share, favorite, etc.
I think part of what makes media definable are the consistency of the media objects’ form. Records are one kind of media because one LP is so much like another in its shape, etc.
YouTube gives web videos that kind of object-y-ness, but those pages on YouTube really work as a media format for anything that fits in the player–from real movies to single images with audio tracks.
So, yeah, YouTube videos are maybe the new 45s more than mp3s have been.