We've seen an entire proliferation of witty, insightful, and occasionally-snarky dialogues with Siri since launch. This, combined with the visual representation of Siri dialogue, accounts for much buzz.

See http://shitthatsirisays.tumblr.com/ for example.

Clearly not the only explanation, but a major difference between the two platforms with interesting PR effects.

It's still a week into the purchase and people are excited about their new purchase. Give it a month or two, and we'll see if people still talk about it. Do people still use Facetime?