Probably because most of the world has a different definition.

1 : an invented fact believed to be true because it appears in print 2 : a briefly stated and usually trivial fact

https://www.merriam-webster.com/dictionary/factoid

Note that these are OPPOSITES.

Ah, what would be the most common incorrect way to use it? Being a non-native English speaker I'm always fascinated to learn new things like this one.

It's often used for a "trivial or minor fact" but by comparison with planetoid (something like a planet but not a planet) or cuboid (like a cube but not an actual cube) or asteroid (looks like a star but is not a star) or humanoid / android (having the form of a human or man while not actually being one), you can see what the 'correct' usage should be.

Trivial fact would perhaps be better as 'factette' or 'microfact'.

I have also heard people use it to describe a "bite sized fact". The "oid" part of the word, for some people, seems to mean "small" or "compartmentalized".

In this incorrect usage of the word, take the following sentence: "Nice little factoid!", could also be written as "Nice little nugget of info!"

If most people are using a word to mean something different, doesn't that mean the meaning of the word has changed?

If I choose to label them "incorrect" for their usage of "factoid", by your own argument I'm absolutely right.

Descriptivists may say yes, but that doesn’t mean people won’t try to stop the meaning drifting to prevent the past becoming inaccessible.

Seems to have happened with "momentarily". Used to mean "for a moment" but now most people use it to mean "in a moment".

I remember being amused the first time a flight attendant said "we'll be taking off momentarily".

It depends which people you are talking to.

Reminds me of Norm's use of the word when reciting a joke about Danny Devito (and Norm's a stickler for proper usage of words)