2ch, where there are many posts praising specific companies or, conversely, disparaging rival companies.
Literal
Specific companies [object-を] praising posts [and-や], conversely rival-companies [object-を] demeaning posts [subject-が] many 2ch.
A noun-phrase description of 2ch (now 5ch), Japan's largest anonymous bulletin board. 書き込み (post/comment) is the standard term for online forum contributions. 逆に (conversely) pivots between two contrasting behaviors. The sentence ends with the noun 2ch rather than a verb — a headline-like fragment.