Everyday I get questions about very small insects like this. Since I cannot see them to identify them I can only make a guess based upon the types of small bugs that infest bathroom. There are two types that come to mind: mites and springtails. In your case I suspect springtails since they have antennae. They seek moisture in bathrooms.
http://en.allexperts.com/q/Entomology-Study-Bugs-665/tiniest-insect.htm
The furniture/carpet beetle (Anthrenus flavipes) is the most common of these bugs. This is a small beetle (usually just larger than a pin head) that is mostly oval in shape with a mixture of black and gray patches. It moves very slowly and rolls over when touched.
http://www.orkin.com/ask-the-orkin-man/tiny-black-bugs-in-laundry-room/
The Mymaridae, commonly known as fairyflies or fairy wasps, are a family of chalcid wasps found in temperate and tropical regions throughout the world. It contains around 100 genera with 1400 species. Fairyflies are very tiny insects, like most chalcid wasps. They average at only 0.5 to 1.0 mm (0.020 to 0.039 in) long. They include the world's smallest known insect, with a body length of only 0.139 mm (0.0055 in), and the smallest known flying insect, only 0.15 mm (0.0059 in) long. They usually have nonmetallic black, brown, or yellow bodies.
https://en.wikipedia.org/wiki/Fairyfly
ANSWER: These are probably clover mites. They are small, reddish mites, about the size of the head of a pin and usually moving around very quickly. Problems with these mites usually occur in the spring, but there are regions that also experience population increases in the fall.
http://www.orkin.com/ask-the-orkin-man/tiny-red-dot-bugs/
Question: We have these tiny little bugs that seem to keep coming in through the windows. Size of a pinhead, move very slowly, round body, sort of black with brown. I find one here and there, primarily on windowsills, so I assume they are coming in through the window. It doesn’t seem to be a full-blown infestation.
http://www.orkin.com/ask-the-orkin-man/tiny-red-dot-bugs/
Question. I just want to begin this by explaining that this is not my bathroom, merely my brothers whose existence I am forced to live with. There are these tiny little insects on the toilet paper in our bathroom. Some so tiny they simply look like a speck of dirt except they move and some are slightly bigger.
http://en.allexperts.com/q/Entomology-Study-Bugs-665/tiniest-insect.htm
Question: We have a bug that is only in our laundry room. It is very tiny and black. They multiply quickly and hang out on the dirty laundry. We also found some in our dog’s biscuits. They turned them to powder.
http://www.orkin.com/ask-the-orkin-man/tiny-black-bugs-in-laundry-room/
Very small. Question: Hi, I went to open some windows in my house today and noticed on some of my windowsills (mostly just the two windows in our above ground basement) there were what looked like tiny brown paint speckles but they were bugs.
http://www.orkin.com/ask-the-orkin-man/tiny-red-dot-bugs/
Curious what the bug is. Question: I have a very small beetle like bug that started in my bedroom but is now everywhere. It is black with white spots and is about a third of the size of an apple seed … maybe even 1/4. I live within 30 miles of Seattle.
http://www.orkin.com/ask-the-orkin-man/tiny-black-bugs-in-laundry-room/
The Mymaridae are the most primitive members of the chalcid wasp superfamily (Chalcidoidea). About 1424 species of fairyflies are grouped in about 100 genera. Of the extant fairyfly genera, Eustochomorpha is the most primitive.
https://en.wikipedia.org/wiki/Fairyfly
what is the tiniest bug
Fairyflies is the tiniest bug. Fairyflies are the tiniest bug.