When AT&T says their DSL service is "perfect for social networking", does this mean ...?

[Black Angel]

This is just complete marketing BS.
Social networking is playing on Facebook and things like that. It's not performance intensive at all. QoS, Quality of Service, is a feature available on most mid to high end home routers. It takes a look at traffic on the network and heading out to the net and decides what's more important. To keep it simple most home devices allow you to mark traffic by a few levels of priority.

1. VPN,VOIP, gaming [Skype and other real time video communications] are usually marked priority or something similar (i.e. - things are NOT delay tolerant - if Skype is a second out of sync or your Xbox live game is, you end up with weird things happening that ruin the user experience)

2. HTTP/HTTPS [Facebook or answers.yahoo.com ] are usually marked bulk or low priority as they don't need to be instant (i.e. they're delay tolerant - if Facebook takes one second to reply to your computer not a biggie)