Enable VoiceOver, then open the Notification Center. Select the 'today' radio button, interact with the scroll area of widgets, and interact with the Social widget. Choose any or all of the groups therein; interact, and you'll find just an unlabeled button (or maybe more than one, it's hard to tell with only VO). These buttons should all have labels. Activating the button will prompt for your post on the selected service, so this is not a big problem, but there should still be labels.