Only Zucker---- can speak to the rationale for Facebook's nudity policies. However, I've seen it speculated that FB's policy is driven by the fact that advertisers don't want to risk the blowback if one of their products were to be seen advertised next to any content religious conservatives will get upset about. Other sites that used to post various forms of semi-nudity have run into this problem.

Facebook itself may also be unwilling to risk losing advertising from conservatively-aligned companies by allowing material that their conservative clients would find offensive.

Religious conservatives can cause tremendous brand damage if a megachurch pastor happens to 'find' something blasphemous that he can use to motivate his flock to up their donations that week. For most companies it's not worth the risk of stirring up these people and it would not be surprising if this is also the case with Facebook.