jQuery Masonry and Ajax Append Items?

It appears that the masonry function expects a jQuery object as its second parameter and not a raw HTML string. You should be able to fix this by wrapping the success callback parameter like so:

jQuery.ajax({
    type: "POST",
    url: ajax_url,
    data: ajax_data,
    cache: false,
    success: function (html) {
        if (html.length > 0) {
            var el = jQuery(html);
            jQuery("#content").append(el).masonry( 'appended', el, true );
        }
    });
});