I am sure that there are some common stock holders in DoorDash looking to offload their stock, but I can’t think of a more certain way to lose all your money than to invest in the common stock of a startup who has just taken a down round.
That all depends on several unknowns at that time. It definitely increases the chances of the holders of common stock to be left without compensation if and when the company sells or there is some other 'liquidity event', but it definitely is not a given.
A down round is simply the issuing of new shares at a different valuation than the previous shares. There could very well be a round at a higher valuation later on, the difference between the previous share issue and the current one could be small, the number of shares issued could be small to the number of shares already outstanding and so on.
Actually, please just keep it off HN. It distorts the discussion in all the wrong ways.