1. This might not be exactly canon, being just a piece of merchandise, not wriiten by any of the cartoon’s writters, but a gal named Susan Ring, and this is aparently the only piece of MLP fiction she worked on.  
2. As I pointed out bellow , Pinkie might be joking/trolling.  
3. Rainbw Dash might have made up her crush on spot to not stand out when other girls describe their crushes (ie a version of the Girlfriend in Canada lie) hence how vague his description is.  
4. Rarity doesn’t tell she has a crush on Spike, just knows (dragon) Spike has a crush on her, and seems to find it adorable.  
5. We don’t know how related Jonathan is with Applejack. His probably from the Apple family, but might be a very distant cousin. (Also, he might not exist in terms of canon seeing this paragraph is the only place he was ever mentioned, connecting to my first point).  
6. These are just crushes - not all crushes are pursued as relationships.  
7. I think this whole bit is meant to be humorous, and doesn’t take itself 100% seriously.
    
   So let’s not overreact here XD
    
   (Then again, people nowdays usually overreact…)

@Zaknel

@Marnssj
Well, I stated before, I think it is possible Pinkie is joking/trolling people, and gave the most controversial answer she could think of, to trigger people in-universe, and out-universe (i.e. us). I mean, she wrote:
I’m going to write it fast, because it’s a confession. There I said it _Sh-h-h-h.

It looks to me at least, as if Pinkie pretended it to be a great secret, and is meant to be shown as trying to hard.

And seeing how at least some people reacted, she succeded XD

But YMMMV.

@Zaknel
what would Fluttershy say if Rainbow Dash started liking Zephyr Breeze?

It would be interesting to see an episode where they deal with one of their mutual friends having a crush on their brother.