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Fads.

Not derogatory, this is what they were called at the time.

The term "meme" for such internet things started in the early 90's.

This is true, but colloquially they were known as fads. At least on YTMND, 4chan, SA, or any other major hub in the late 90s/early 00s

According to Wikipedia :

The word meme itself is a neologism coined by Richard Dawkins, originating from his 1976 book The Selfish Gene.[12]

Fads was a strictly YTMND term. On 4chan they were generally called memes, on SA they were image macros.