It seems fairly understandable even without financial knowledge?

1. Facebook creates a shell company.

2. The shell company borrows billions of dollars, and builds a data center.

3. Facebook leases the data center.

4. The fact that it is technically only a four-year lease with only one possible tenant can conveniently be ignored, as Facebook assumes essentially all possible risks. The shell company could only possibly lose money if Facebook itself goes under, so the lenders can treat the loan as just as reliable as Facebook itself.

5. Because Facebook technically only has a four-year lease, it can pretend it doesn't actually control the shell company: after all, it can always just decide not to renew the lease. The fact that is assumes essentially all possible risks can conveniently be ignored, so Facebook can treat it as a separate entity and doesn't have to treat the debt as its own.

So the lenders are happy because there's no real risk to them, and Facebook is happy because they can pretend a $27B loan doesn't exist. It's a win-win, except for the part where they are lying to their shareholders about not taking on a $27B loan.