Beyond speakers, this is great advice for anyone whose public work you appreciate that isn't a major celebrity. I've sent questions and compliments to creators of all kinds of work I enjoy and more often than not gotten a genuinely thoughtful and appreciative response back. It could be a small website creator, a researcher in a field outside my own, an author, or an open source project maintainer.

My sense is that not many people do this and anyone who is not mega-famous rarely hears from strangers who like their work.