The context is he was 19 and the quote's from an instant message conversation.
I've never seen an "explanation." It seems self-explanatory. I haven't seen an apology either, but this was in the New Yorker:
When I asked Zuckerberg about the IMs that have already been published online, and that I have also obtained and confirmed, he said that he “absolutely” regretted them ... Zuckerberg’s sophomoric former self, he insists, shouldn’t define who he is now.
