This seems to be a completely empty Facebook comment. Comments consisting solely of newlines or spaces are not allowed, but a blank comment can be made by inserting a soft hyphen (via ALT 0173). However, unlike the comment in the link, a soft hyphen comment is still "selectable" by dragging over it, and leaves a trace in the html.
Blank soft hyphen comment:
<span class="UFICommentBody"><span>­</span></span>
The empty comment:
<span class="UFICommentBody"></span>
Some tutorials suggest typing @[0:0: ], but Facebook inserts a space between the "@" and "[".
So how would one go about doing this?
EDIT:
Typing "@[0:0]" from the Facebook Android App results only in a "0". Typing "@[0:]", however, seems to produce no comment whatsoever, but perhaps there is a workaround?