As Jack is just a nickname for John, it stands to reason that it would be popular; John and its variations in other languages (Juan, Sean, Hans, Giovanni, etc.) is probably *the* most popular male name in the Western world, over time.
For anyone with an Irish surname who might be thinking about Oliver, you might want to reconsider, especially if you're Catholic. For an Irish kid, being named Oliver is akin to a Jewish child being named Adolf -- very bad historical association. (And for anyone not up on Irish history, the "Oliver" that causes the problem is Oliver Cromwell.)