No, they aren't. True, if it were not online, Facebook's involvement with selection of which provided content to promote might make them a publisher (it's kind of hard, because both bookstores and other distributors and publishers, offline, actively select content in very biased ways, but face very different liability regimes). But while you reference “§230-protected content”, you fail to understand the operative language of § 230: “No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider.” Note that there is no “unless, as a provider, they choose to promote that content in a ‘clearly biased’ manner” caveat. Just a bar against the provider being treated as a publisher of content that originates from someone else.