Councillors cannot as though they are the council, e.g. "We will build a new park" appears to speak for the council, but "I have proposed a new park" is fine.
However we dress it up, we are politicians, and are entitled to opinion, and if councillors disagree with the bus shelter then they should say so (it will also help people know who to vote for or not in the future).
Councillors should, however, hold themselves to a high standard of public debate, not only for the council's benefit, but also their own standing.
Social Media guidelines should ensure the councillor issue only factual information, or (voted upon) opinions of the council, e.g. "the parish council object to the new road" needs to have been voted upon and not just be the chairman's opinion.