With pr third party votes are bad for socialists

The problem with fptp is that there is only one choice to (tactically) keep out the main centre-left socialist party. With fptp there is a monopoly on anti-socialism. With pr there are many different parties which oppose socialism. In fact under pr all third party votes are bad for the leading socialist party. With fptp third party votes do not hurt the socialists and so we can say that they fall in favour of the socialist party. With fptp third party votes are favourable to the socialists whereas with pr third party votes are bad for the socialists.