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".
2012年9月6日
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"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.