How popular were boarding pikes? Did navies use them often, merchants, or pirates? When did their use start to decline, and then eventually become obsolete?

See this post: Which weapons did pirates use during the Golden Age of Piracy (1650-1730)?

The concept of a stick with the point on the end of it as a weapon is real old - and they didn't abandon it by any means in the 18th century. While you could use it on the offense, the boarding pike was even better on the defense - especially when you have a defense set up. The pike is great when there are presses of men, since it will be hard for a target to get out of the way of your pike. Also, if the enemy is coming over the railing, get them then. Better yet, if there is boarding netting up (basically, netting that is strong enough to resist being pulled down and hacked away for at least a little bit, slung around the ship's bulwarks (the walls that extend from the deck around the ship) and up several feet to make it hard enough to climb/jump over. With the netting up, you could stab through the gaps at your enemy that will likely not be in the position to defend themselves. The early 18th century marked a time when boarding netting was somewhat in their beginning use on civilian ships - the British Navy (and most navies overall) definitely adopted them by the middle of the 18th century.