Best friend didn't say happy birthday?

For what it's worth, happy belated bday from a random interwebz stranger, kiddo.

Unlike whatever impression the media might engender, this is how most relationships end over the years - going separate ways, becoming different people, spending time with new acquaintances. Given the rapid pace of changes during youth, there shouldn't be too much time to get settled. There's no need to make a fuss anent it, unless it's for the sake of holding onto one you truly wish to preserve, because that will be incredibly difficult.
You already stop being friends informally once interaction boils down to awkward glances on the sidewalk or passive greetings; and you can resume your friendship should the passion for it arise once more, from mutual interest, experiences or necessity.
If you're going to get offended over breaking some convention or another, you're within your rights; however, since your starting point honestly doesn't sound famous, you might come to regret that choice.