He hardly ever looks here, so this is the least effective way of reaching him.
His email address: <jephco at mac dot com> is on the comic's "
contact" page, and I believe that works; he says he gets a lot of email, so he doesn't necessarily answer personally - even to messages he appreciates (I've never had an email reply from him). There's also a link to
his Tumblr, which is another good way to be reasonably sure your message gets read; for general queries, he batches up selected answers into QA posts on the Tumblr. I guess
Twitter would also work; but this feels unnatural to me, but that's my problem (I find a lack of clarity about what's private or public on Twitter and Facebook, so try to avoid them).