A Diigo help page says you have to have 2 friends to post public stickies. I don't know if it means 2 people following you, or 2 reciprocal follows, or what, but here's the actual text from the help page:
"Public highlights and sticky notes should be done with care. To minimize graffiti and spam, we have raised the bar for making public highlights and sticky notes: you need at least two friends in the Diigo community to do so."
...and that's all it says.
I, too would like the ability to post public stickies, but so far I have 0 friends on here.

Alright, I've followed you both, and after I did that I also created a friend list and added you to it, as Diigo suggested that I do. Maybe that's the missing step.

nope, I still can't. guess I need 3 more.