The best way to contact me is via email at email@example.com.
If you prefer, you can friend me on Facebook and message me there.
If you would like to ask me a question that I can answer publicly, you can use my “ask me anything” repository on GitHub or my Tumblr “Ask me anything” form. I will be notified by email (via GitHub or Tumblr, respectively) but will respond publicly (if I choose to respond). Content is not shared between the GitHub repo and Tumblr, so e.g. a question posted on GitHub will not show up on Tumblr.
You can also send me anonymous feedback (an idea I got from Luke Muehlhauser and Gwern).
I try to respond to everyone who contacts me, but for various reasons I might not; feel free to ping me if I don’t respond. Also see “I emailed my customer asking multiple questions. Their reply addressed only one. What is the polite way to point this out?”; i.e. I will try to respond to every question or point, but sometimes I might not (for whatever reason). If you really want an answer, feel free to re-ask the question/restate the point.
See the page on account names for account names I use on various websites. You can try to contact me through those sites (if they offer a messaging feature) but other than Gmail/Facebook I can’t guarantee I will check them often.
You can stalk me online (e.g. by following me on Quora or Facebook or by reading what I have on this website or my contributions to other sites). I also don’t mind if you ask questions about me on Quora, if you want to passively contact me that way, though I don’t control what Quora allows.