When the hormone insulin is released into the bloodstream, what form of cell-to-cell signaling is being used?

The form of cell to cell signalling that is been used is ENDOCRINE.

Explanation:

There are four ways by which cell signalling occur in the body, these are: endocrine, paracrine, autocrine and direct signalling.

Endocrine signalling is the biochemical process by which hormone secreting glands release hormones into the blood streams and that hormone has effects on target organs that are located far away from it. Endocrine hormones are usually stable and their effects usually last for long time.