A missing dimension is that group-based systems may be vulnerable to knowledge leaking to other individuals in the group.
To take an example, even with privacy barriers up, it is clear that companies target advertising and marketing by IP address. Technically, because the IP is an ISP provided address, individuals themselves cannot be identified, but Google et all know the approximate location of the users and can use this to target based on group-based interests. But what happens is that information leaks across the individuals in the group. Adverts that one of the individuals sees, will contain information and clues as to what neighbours have been searching for. So although the advertiser is technically acting anonymously, anonymity may be breached as other individuals in the group speculate on which of their neighbours is looking for certain items - for instance health related, new jobs etc.