But is it a blow? I'm not sure that the convenience of integrated email in Facebook would outweigh privacy concerns. Facebook already has rights to all the pictures of my kids I post there. That's why I post most pics to Picasa, which then notifies Facebook. They'd have rights to my email contents too? And even if they don't have rights, they know who I've been emailing with, both inside and outside Facebook. Fundamentally, their business model is predicated on users' online promiscuity.
While teenage communication behaviour is often used as a predictor of future communication culture, you have to balance that with life phases. Most teenagers (certainly those with time to burn on Facebook) don't have a professional life. Their life is all about social networking, online and offline. I'm not convinced they'll carry those same traits into their professional lives.
Will Facebook become more than a 'keep in touch with friends' tool in mainstream adult culture? I'm not convinced. Facebook taking on email is a good, sensible progression - I have several friends & family whose email I don't know because I just contact them via Facebook. But as anything more than a convenience?