I'd love to have actual distributed transactions that could scale indefinitely and not create availability issues. We actually get a steady stream of user complaints about inconsistencies between counter caches and what appears in results. Worse is the inconsistencies that can happen between graph edges that you want to partition in two different manners (eg. following vs followers).
Likewise, consistency's more important in facebook-y scenarios than you'd think. The canonical example I've heard is that "defriend my boss" MUST always be seen before "post 'I'm quitting!'"
