沙发 is the first comment or reply. 板凳 is the second one.

网络用语沙发的意思——某个人在博客或空间发表了一篇文章，然后第一个你去评论它，这个是表示你所占有位置。

インタネトの言葉です、第一人のこたえで。　それも中国お宅たちの言葉、ほんとにつまない。。。

Agree wang yang's answer, and there are two other explanations about this word. The first one means you'll sit on the sofa to read a excellent post in forum, and the second one is from the word "首发贴", "首发" sounds like the "so fast".

"沙发 " belongs to the postmodern adjectives, transliteration from English "SO FAST" network terminology, it is commonly used in the blog posts to respond quickly or fast response.