border-shit: a new CSS property emerges
Twitter and Facebook: We save a request and, maybe, a byte or so, but our tiny arrows look shitty, with the Facebook's one being the shittiest.
Google: We have a sprite and our tiny arrow doesn't look shitty. We don't like border-shit, it is not standard, anyway.
Вот и dirty.ru тоже сверстали себе стрелочку таким образом.