You are correct that mass manipulation is a critical issue. However, this vulnerability is shared by any system reliant on voters, including the representative one. It is not a unique flaw of direct democracy.

So there are three issues we're talking about in this context:

1. Reps are also uninformed.

2. Social media manipulation of the populace (or, generally, propaganda).

3. Concentrated influence on a handful of legislators.

Direct democracy eliminates the third vector.

Furthermore, the stakes and incentives for corruption are vastly different. A lobbyist gains far more from corrupting one senator who decides for millions than from swaying individual voters. The return on investment for corrupting concentrated power is orders of magnitude higher.

Even if propaganda shapes opinion, the resulting decisions still represent the people's will at that moment. Representatives can betray even that will for personal gain, adding another layer of distortion between what people want and what they get.