Porn Killing Religion!
Well, for one, the increase in internet usage can only be a huge percentage, because the WWW didn't even exist before 1989. You might as well say the steady decline of the VCR and the tape cassette is the reason for the loss of religious faith today.
Next, conservatives blaming pornography for society's ills has been around a long, long time. Just watch that video, and ask yourself: if that were re-filmed, word-for-word, changing 'magazine' for 'website', would it surprise you today? The same rhetoric has been around since the 19th century -- If The Onion were published and shipped through the mail in its current form in the 1920s, they'd have been arrested for sending obscene matter through the mails. The presence of this material in easy reach today is a symptom of lessening moral confusion than it is the cause of it.
The problem is, you can't trust surveys about religious belief - people report how they wish they behaved. The fact that the self-reported religious belief is going down is simply that people are less afraid of seeming less religious.
The article I linked does make this connection, though, before going off the rails into correlation issues. The internet allows for communication and the spreading of free information, which allows people the opportunity to step outside the rigid restrictions of their close environment. This gives people an outlet where they can actually express their lack of religious feelings, rather than feeling pressure from all the people who are lying on religiousness surveys.
Sorry, pornography: I wish I could say you killed religion, but it turns out you're the symptom, not the cause. Just keep dressing this lady like a nun, and then undressing her like a nun, and then I won't complain.