I recently came across this picture and would like to replicate the effect. I have tried Illustrator's halftone effects, but the dots are too tidily arranged and I prefer it to be random sizes and positions.
I literally see myself drawing this manually. I don't deny the fact that (Just as Lucian said) this might be done in some other software or with a plugin but just look at it.
This can totally be done with a custom pattern brush (just repeating circles) for the main shape and hand drawing some of the particles.
When I initially said drawing by hand I didn't mean it literally. Just create a custom brush with dots, use the Pen Tool to give the main shape and apply that brush. Then simply click/tap with a mouse/tablet-pen to create the rest of the dots.
It is best to do this initially in Photoshop. Then convert to bitmap (50% threshold). You can then import the bitmap into illustrator and change to any spot colour you wish.