You'll make your real friends

In the example:
What does this sentence mean? :
“You'll make your real friends,
those cunning folk use any means to achieve their ends.”

example:
"Oh, you may not think I'm pretty,
But don't judge on what you see,
I'll eat myself if you can find
A smarter hat than me.
You can keep your bowlers black,
Your top hats sleek and tall,
For I'm the Hogwarts Sorting Hat
And I can cap them all.
There's nothing hidden in your head
The Sorting Hat can't see,
So try me on and I will tell you
Where you ought to be.
You might belong in Gryffindor,
Where dwell the brave at heart,
Their daring, nerve, and chivalry
Set Gryffindors apart;
You might belong in Hufflepuff,
Where they are just and loyal,
Those patient Hufflepuffis are true
And unafraid of toil;
Or yet in wise old Ravenclaw,
If you've a ready mind,
Where those of wit and learning,
Will always find their kind;
Or perhaps in Slytherin
You'll make your real friends,
Those cunning folk use any means
To achieve their ends.
So put me on! Don't be afraid!
And don't get in a flap!
You're in safe hands (though I have none)
For I'm a Thinking Cap!"