What would be the first use of the noun 'longboarding' for skateboarding, surfing or skiing?

Solution 1:

The Oxford English Dictionary can shed some light on this. The earliest attestation is 1950 for the ski sense:

Although she readily admits she is still a novice, she should handle her longboards with considerable dexterity if she improves as rapidly in skiing as she did in bowling.
_{Daily Inter Lake: Dec 24, 1950}

And 1970 for the surfboard sense:

At left is Dennis Heussner, captain of the Australian surf team and national longboard champion.
_{Santa Cruz Sentinel: April 2, 1970}

The OED explains why it may have taken so long to see longboard used in the surfing sense:

The surfboards so named were originally the ordinary kind and hence may only have received this designation after the appearance of the shortboard. In the following quotations long and short are probably simply ad hoc descriptions of especially long and short surfboards:

Duke Kahanamoku built his great 16-foot hollow redwood board along about the same time [1910]... Duke did some of the most beautiful riding I have ever seen on his new long board.

About 1903 we used a short board a few feet long, rather thin and wide, like a wash board.

Both of those quotes are attributed to a 1935 publication called Hawaiian Surfboard.

(It's also worth mentioning that there are some hits for longboard that date back to the 1800s, but those refer to especially long moldboards, used in certain tractors.)