Short answer. It can be. Truth alone is not a complete defence under Philippine law: a defamatory imputation is presumed malicious even if true, unless you can show a good intention and a justifiable motive for saying it, or the statement falls within one of the privileged situations the law lists.
What the law says
Every defamatory imputation is presumed to be malicious, even if it be true, if no good intention and justifiable motive for making it is shown
Revised Penal Code, Article 354 — Requirement For Publicity. Read the full provision →
Why truth is not automatically enough
This surprises most people, and it is the single biggest misunderstanding about Philippine libel. In many countries truth ends the case. Here the law starts from a presumption of malice the moment a defamatory imputation is published, and expressly says that presumption survives even where the imputation is true. The burden therefore lands on the person who spoke or posted. Showing that the statement was accurate answers only half the question; you must also show why you published it — a good intention and a justifiable motive. Airing a true but private failing purely to humiliate someone is exactly the conduct the presumption is aimed at.
The two situations the article carves out
The article names two categories where the presumption does not apply. The first is a private communication made by one person to another in the performance of a legal, moral or social duty — reporting a concern to the person who has the power to act on it, rather than broadcasting it. The second is a fair and true report, made in good faith and without comments or remarks, of judicial, legislative or other official proceedings that are not confidential, of statements made in those proceedings, or of acts performed by public officers in the exercise of their functions. Both carve-outs are narrower than they look: adding your own editorial sting to an official report can take it outside the protection.
What still has to be proved against you
This provision deals with malice, not with the rest of the offence. A complainant must still establish that the imputation was defamatory, that it was published to a third person, and that the person defamed is identifiable — even if not named. If any of those is missing there is nothing for the presumption to operate on. Note too that where the subject is a public officer and the imputation concerns the discharge of official duties, the law gives more room to speak, and proof that the charge is true can be admitted. Private grievances about private persons get no such latitude.
If you are on either side of this
Anyone who has been threatened over a post should preserve the original wording, the timestamps and the audience settings before anything is deleted, since the scope of publication matters. Anyone considering a complaint should act promptly, because libel carries a short prescriptive period. Bear in mind that defamation posted online is separately punishable as cyber libel under the Cybercrime Prevention Act of 2012 (RA 10175), with a heavier penalty than the printed form and a longer window to sue, so the medium you used changes your exposure considerably. These are general rules; how they apply to a particular message depends on its exact wording and context and should be reviewed with counsel.