As someone who initially encountered effective altruism, and then Less Wrong

Actually for me, it was the reverse, I encountered LessWrong and then later "effective altruism".

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do you have a perspective on how, or how much, Less Wrong has amplified the success of effective altruism as a social movement within the last couple of years?

Not really. It seems quite clear that LW has definitely amplified the California EA community around things like existential risk reduction, Friendly AI, cryonics, anti-aging, etc. And it's certainly helped build a community. But other things like Peter Singer, GiveWell, and CEA were necessary for creating and maintaining an EA movement.