For example this is the current law covering the offence of harassment under the non fatal offences against the person act 1997
"10.—(1) Any person who, without lawful authority or reasonable excuse, by any means including by use of the telephone, harasses another by persistently following, watching, pestering, besetting or communicating with him or her, shall be guilty of an offence.
(2) For the purposes of this section a person harasses another where—
(a) he or she, by his or her acts intentionally or recklessly, seriously interferes with the other's peace and privacy or causes alarm, distress or harm to the other, and
(b) his or her acts are such that a reasonable person would realise that the acts would seriously interfere with the other's peace and privacy or cause alarm, distress or harm to the other."
Commenting on another actions to other people and not directing it towards them certainly doesn't reach the threshold.
And even commenting directly on there behaviour on their own social media pages would not unless done in a manner intended to seriously interfere with their peace or privacy.
Going to be very hard to argue you're interfering with someone's privacy by commenting on something they publish for public consumption and financial renumeration