Can the Bitcoin network be used for cracking?
Short answer: no.

Long answer:


Blocks are exactly 80 bytes long. When have you ever seen an 80 character password?
Blocks start with a bunch of null bytes in the version field. Again, when have you seen that in a password?
Each miner is mining a different block. You don't know the merkle root of what they're mining because that contains a hash of a secret address.
Bitcoin uses SHA256(SHA256(x)) as a hash function, which is not very common in password hashing.
Miners won't publish their results unless they solve a block. They only solve a block if it starts with 12 zeros (at time of writing). The odds of your hash starting with 12 zeros is 0.000000000000003034%.