ON the internet / This is going ON my blog for sure!
We use “on” with stuff on screens. What’s on? (Qué echan -TV, movies/films, theater/theatre, concerts…)
AT + web address: but sometimes we use AT, as a pointer, deictic, pointing at a geographical spot in space! Check this out at talkingpeople.net. You can also use ON, the regular preposition.
IN is possible, when you feel inside the website, blog, for instance because you enter as an author or administrator. Or when the place is huge and you spend time there, surfing its pages. But this is subjective.
The safest option is ON. And AT when giving the address.
Are you ON facebook? (So this is why, OK? It’s a screen)
Subjective: but then, when we meet up on facebook, hahaha, then we might say, “I’ll see you in facebook!” as in “I’ll see you in class next day”.
Are you ON email?