Title text: I've seen advertisers put their URLs on chalkboards, encircled with a DNE. They went unerased for months. If you see this, feel free to replace the URL with xkcd.com.
DNE stands for "do not erase", and is commonly used on school whiteboards to let the sanitation staff and other teachers know not to erase that particular area of the board. DNE circles often encompass important information such as test dates or the teacher's name.
It is easy to see how leaving things marked with DNE can become automatic for anyone often erasing boards, so that they don't notice what the message actually says, or reflect on whether the DNE-marking is reasonable for it. Cueball's dissatisfactory note is not very subtle, but Randall notes that advertisers have successfully used the same tactic for less conspicuous URLs to their sites.
In the title text it is pointed out that many advertisers would just write down a website address, circle it, and then add a DNE marking. It also kindly asks you to replace those messages with xkcd.com to get some free advertising.
- [Cueball is in an empty classroom writing on the whiteboard. In the top right corner in large print is written "Fuck This Place!." It is circled, and underneath he is writing "DNE".]