About seven months ago two other people and I rented a house and all signed an eleven month lease. After about a month, one of the three of us moved out and another tenant moved in. The landlord sent us a new lease to sign, but it never got back to him. Recently, my two roommates decided to bail on the house and left me with no choice but to move out, essentially abandoning our house. My question is, If I haven't signed a lease (since the first one which I have been told was broken when the first roommate moved out), am I still legally bound to the landlord in any way?

Yes, your original lease is still valid. You may have been told that the lease was broken but it wasn't legally ended until the landlord acknowledged it in writing. Signing a new lease would have served that purpose, but since a new lease wasn't signed the previous lease is still in effect.

Most likely the original lease contained a clause saying that each of the tenants was jointly and severally liable for the lease. That means that the landlord can sue one, two or all of you for the entire amount. If you are the only one who gets sued then you can sue your former roommates for their share. Likewise, if one of the others gets sued then you would be responsible for reimbursing them for your share.