Habermas is known for his notion of the Public Sphere and his Theory of Communicative Action/Reason. The Theory of Communicative Reason is not based on universal reason but (related to Public Sphere) through actual interpersonal communication. This implies the optimism for progress – his belief in the practical/pragmatic potential of human reason through interaction and communication.
Part of this optimism in Modernity has to do with the emergence of the Public Sphere in early Capitalism, a sphere in which intellectuals, the media, writers, etc. could exchange ideas and critique the powers that be. However, the Public Sphere eventually became mass produced and blended too thoroughly with commercialized mass communication, namely in the postmodern era. The critique of those in power is lost in the mix.
However, Habermas thinks the sphere could be resurrected; hence his belief in Modernity over Postmodernity, which he found (like the sphere lost in the mix) to be too ambiguous. Habermas (and others) argue that with Modernity, there are more staunch positions, thus a more defined position and place for a Public Sphere (a sphere of critiquing those in power). In Postmodernity, the public conversation is too muddled; therefore, Habermas argues for an approach that is more aligned with the earlier projects of Modernity.
Habermas favors radical democracy; thus his optimism in Modern democracy. There is certainly a democratization in Postmodernity - such as social media providing more means for voices of dissent and critique. And while Habermas would likely acknowledge this, he has even more optimism for a more defined Public Sphere. Habermas is in favor of radical democracy, so he would embrace social media. But, stemming from his Marxist readings, he is careful to discern how any sphere might be manipulated by those in power - via commercialization and so on. In this sense, the Public Sphere must somehow be beyond the influence of state or corporate interference.