Google provides a new way of communication.
1, You type in a message on some crawlable random webpage with an interesting distinguishable keyword.
2, Ask the other party to search for the keyword couple of days later.
Here you go! The key here is to make your message as plain as possible but your keyword as strange as possible. That's what I did in this message. I plan to implant the word "arthurchanlialastica" here. (As I just did.) Since this is a new word (searching in Google return no message), it's IDF should be fairly high and causing the retrieval engine rank it high. I expect when I search "arthurchanlialastica" Then this message will come up.
I also expect this will be "quite anonymous". Since other than "arthurchanlialastica", all the information is quite plain. So other parties without a search engine will not be able to spot my message. What happens here, "arthurchanlialastica" become a "radio frequency" which I could use to communicate. Sounds like an interesting idea.