Slings are low cost and effective weapons. I am not sure about their popularity among shepherds, however a sling would be an ideal choice as it is portable and ammunition is provided by the environment, should prepared stones be unavailable. The Aztecs had a longer version of a sling that was alleged to be accurate at a hundred yards. These slings could even damage soldiers in armor. I had a history teacher bring one to class and demonstrate. It was an impressive exhibition. In practiced hands a sling is an outstanding weapon.

King and Stager strike again (get it?!)! From their lovely Life in Biblical Israel:

"The sling, a weapon of great antiquity, was used by both warriors and shepherds (and, without a doubt, by all the youngsters). Almond shaped sling stones and mud sling stones go back to the Neolithic era. A hollow pocket (thong) of cloth or leather large enough to hold a stone was attached to two cords. The stones found at Lachish were smooth and rounded, often of flint, measuring six or seven centimeters in diameter and weighing about 250 grams... They were carried in a bag suspended from the shoulder. Placing a stone in the sling's pouch, the slinger held the ends of the cord, whirled the sling in the air and then released one cord, propelling the missile with amazing accuracy. The left-handed slingers of Benjamin were legendary: 'Every one could sling a stone at a hair, and not miss' (Judg. 20:16)." (pg. 228)

They go on to describe David's encounter with Goliath as well as various reliefs depicting assaults on places like Lachish where the sling was used in battle. Ultimately, it was a rather easy weapon to acquire (or even make), and the ammo was readily available in abundance.

So yes, common indeed.

Edit: typos, because I didn't proof read.

You may also be interested in our FAQ section on The Sling as a Weapon.

Also, there is contention as to whether or not David was originally the person credited with killing Goliath.

(2 Sam. 21:19) - "And there was war with the Philistines again at Gob, and Elhanan the son of Jaare-oregim the Bethlehemite killed Goliath the Gittite, the shaft of whose spear was like a weaver’s beam."

Some scholars believe the slaying of Goliath was later attributed to David in the historical record in order to bolster the legitimacy of David's kingship.

However, 1 Chronicles 20:5 says, "And there was war with the Philistines again, and Elhanan the son of Jair killed Lahmi the brother of Goliath the Gittite, the shaft of whose spear was like a weaver’s beam." This is, supposedly, the correct answer; namely, that Elhanan killed Goliath's brother and not Goliath himself.

Follow up question, I am not too acquainted with the story of David vs Goliath but what was Goliath? Was he a man or some sort of beast like out of Greek Mythology.

Husky 54 says the sling was used by warriors as well as shepherds but would David's sling be recognized as a relatively weak weapon? (Thus matching his apparent mismatch against Goliath)?