I think I'd start with forcing advertising transparency on any social network (and probably search engines too -- actually, make that any site with advertising)

I think I'd start with forcing advertising transparency on any social network (and probably search engines too -- actually, make that any site with advertising). And then force any company paying for political advertising to list every single one of their donors publicly. Chuck an unlimited personal liability clause in there for the company directors, so they can't just wind the company up to avoid any fines.