There are a few situations where one could yell this phrase, but the main thing is to convey sarcasm. For example if someone you know says something that seems utterly useless in a tone where they're trying to sound like it's the news of the year (and it really isn't), then you'd say this phrase in reply. See alert the media.
(instant messenger convo)
Joe: holy shit dude omg i just asked out sally and she said yes *random emoticon* oh man shes so hawt
Jack: ZOMG ALERT THE INTERNET!!!~ lol dude i dont care shes prolly gonna hump u and dump u rofl
Joe: stfu *angry face* ur just jealous cuz u didnt get her u have no balls
Jack: maybe i should alert teh internet that ur a total douche lol