Huh, it seems a lot of people are quite interested in the issue, as there are sixty comments (admittedly, a lot of those are mine, but anyway.)
It was pointed out that crossposting might undermine pseudonomity, not in a general sense (newcomers changing the site culture), but in a specific sense (people's friends crossposting, and then the crossposter's FB friends coming over to DW, and then possibly finding them.) In other words, say that A and B both use DW and are linked here (through access/subscription). A blogs pseudononymously at DW and wishes to keep their RL identity and their pseudonoymous DW identity separate. B does not blog pseudononymously at DW, and so crossposts to FB. What if B's friends on FB decide, prompted by this, to go to DW? Then, it is possible that they might find A via B's profile or by reading B's read-list or some other means, thus undermining A's pseudononymity.
While this is an issue, I don't think this is one which should be solved by banning B from crossposting to FB, or otherwise discouraging them from doing so. The same problem would exist if B had a WP blog which linked to A's DW journal, and B was crossposting their WP blog to FB. It's an issue which exists when someone who is pseudononymous and someone who is not are publicly linked to each other: I am not sure how it is generally solved in other situations. (I personally solve it by keeping a strict separation, and also, on LJ and DW, not posting publicly RL info: location, birthday, etc.)
There was also the important point of how social networking sites are generally not monocultures, and how multiple norms may exist on other sites. In some parts of LJ, it is the norm for people to use IRL names, and not in other parts. I think this could occur on DW, and wouldn't necessarily be a problem either. There was also a call for more non-LJ users on DW
Also, I was thinking about Tumblr, and actually, it has some FB integration. This, in my limited anecdotal knowledge, has not undermine Tumblr's own culture, or made it difficult to blog pseudononymously there, as I do. In fact, I would say that Tumblr minimizes the above issue (undermining of pseudononimity by visible social media links) by allowing people to hide on their blog their lists of followers/followed.