Current physics concepts suggest that the lower limit for the mass of a Black Hole is about 22 micrograms. According to Stephen Hawking's theories, however, such an entity would be inherently unstable, and would 'evaporate' almost immediately after its creation.
Micro Black Holes gained attention recently with speculations that the Large Hadron Collider at CERN may now be sufficiently powerful to create them as a side effect of some collisions. 1). However, as mentioned above, Hawking theory proposes that they would instantaneously self-destruct. It has also been pointed out that cosmic rays, which permanently bombard almost all regions of the universe at all times, can also have the energies necessary to create Micro Black Holes.
It should be noted that if Micro Black Holes can persist for any length of time their potential to attract other masses and therefore grow irrevocably, would be extremely low. Using the example above, think of the gravitational attraction exerted by a mass of 22 micrograms.
In summary, it's currently not known if Micro Black Holes (or their remnants from 'evaporation') are common in the universe - or if they exist in Nature at all (for any appreciable length of time).