In the early days of Israel, there were communities of prophets, sort of like monks. They were into ecstatic worship. Their prophecy was not of the commissioned-by-God-to-say-this sort. Those with a unique commission sometimes were associated with these communities, sometimes not. Saul met a group of communal prophets (I Samuel 19) and got the twitches in their company, but Samuel seems not to be involved in their activities. On the other hand, there's the story of Elisah leading a group of them in building huts for themselves (2 Kings 6).
But what intrigued me was the relationships between the major prophets themselves and how God seemed to call them in clusters, usually when times were bad and the people were turning to evil. In fact, the thickest lot of prophets came in a great, overlapping cluster toward the end of the kingdom of Judah.
Anyway, in the days of Ahab and Jezebel, the northern kingdom of Israel saw first Elijah, then Elisha, lead the opposition to their wicked ways. But the literary prophet Obadiah was also active in the court of Ahab.
Later on, the careers of Hosea, Amos, and Jonah all overlapped. Hosea was from Israel; Amos was sent to Israel out of Judah; Jonah seems to have been a native of Israel.
As Israel reached the tipping point, God's focus seems to have shifted to Judah and Jerusalem. Isaiah and Micah, whose spans overlapped Hosea's, were both natives of the southern kingdom, who saw the northern kingdom fall. They were part of the desperate defense of Jerusalem and saw its (temporary) deliverance. By the time of their deaths, Judah was sliding toward idolatry again.
As Judah underwent its last revival (under King Josiah) and then its last slide into chaos and subjugation by Babylon, prophets came thick on the ground. Ezekiel was taken to Babylon in the first deportation and was active there all through the subsequent conquest and early struggles. He was contemporary with Jeremiah, who was a friend of Zephaniah and Habbakuk. Daniel was a younger contemporary. Nahum was active in the waning days of Judah, too.
After the Babylonian captivity, as Ezra and Nehemiah led the rebuilding of the temple and the re-establishment of Jerusalem, a trio of prophets were all active: Haggai; Zechariah; Joel.
The last prophet, Malachi, was truly the last until John.
So, my observations are these. 1) God sent prophets in times of great unfaithfulness, not to condemn, but to call the nation back to himself; when the nation rejected their message, it stood as a witness against them. 2) The tougher times got and the worse people behaved, the more prophets God would send, until a true tipping point had been reached; then, silence. 3) God may give up on kingdoms and societies, but he does not desert his people; he will be with them even if all the props they thought essential are taken away.