Labels on wordpress comments are hidden?

It’s related to your theme. When you don’t enable Social Commenting, your theme sets display property to block for the labels in comment form (for which display property is none initially) on page load.
When you enable Social Commenting, plugin tweaks the comment form position to include it in tabs that’s why theme can’t locate comment form and its labels on page load and can’t set display to block.
To fix it, place following code at the end of “wp-content/themes/CURRENT_THEME/functions.php” before ?> (if ?> is not there, simply place the code at the end of the file) and save the file back. CURRENT_THEME is your active theme/child theme

function heateor_ss_custom_css(){
	?>
	<style type="text/css">
	#commentform label{
		display:block
	}
	</style>
	<?php
}
add_action('wp_head', 'heateor_ss_custom_css');

Oh, thanks a lot for the code and explanation!

BTW, I had to use display:block !important;, whitout the !important did not work (well, maybe because I just added it to the child theme css file… instead adding it to custom css function… I guess that would prevail and important not needed)

Great