So how is libel committed?
Some important features of libel under the Revised Penal Code are that (a) Libel is a public and malicious imputation of a crime, vice, defect, etc. and (b) It is a crime committed by means of writing.
To prosecute a crime under the Revised Penal Code, all the necessary ingredients of the crime must be proven.
The following are:
(1) There must be a malicious imputation of a crime, vice, defect, or an act or omission, condition, status or circumstance, tending to cause the dishonor, discredit or contempt to a natural or juridical person, or to blacken the memory of the one who is dead. 
(2) Libel may be committed by means of writing. 
(3) Must be done in public. This means that someone (3rd person), other than the victim (private offended party) was able to read or see or know regarding the libelous material.
(4) Identity of the victim. This means that the victim must be identified as the subject of the libelous matter, that his reputation is the one targeted.
So what were provided under the Cybercrime Prevention Act (R.A. 10175)?
This law makes libel as defined in Sec. 4(c)(4) as punishable by one degree higher than that what is provided under the Revised Penal Code.  Libel is one of the content-related offenses that is committed through use of computer or other similar means (law foreseeing the possible inventions related to computers). 
Therefore, when you post libelous statements on facebook, and all the necessary ingredients of libel are present, you may be prosecuted under the CYBERCRIME PREVENTION ACT, imposing a HIGHER PENALTY.
In other words, libel is penalized under the Revised Penal Code, but, when libel is committed online, then the perpetrator suffers a higher penalty.