The union at the centre of the Southern Railway dispute is to make an official complaint to Twitter over an attempt by the train company to encourage passengers to contact it over the impact of strikes.
The train operator took out newspaper adverts headlined Let's Strike Back, including the twitter details of the Rail, Maritime and Transport union.
The adverts said the union will not listen to Southern, "but they may listen to you".
Most people who responded criticised the company, rather than the union.
The RMT said it believed the advert amounted to "targeted harassment", which contravened Twitter's regulations.
Southern has told the RMT it will press ahead with changes to the role of conductors if the union does not reach agreement by noon on Thursday.
It will "regretfully proceed " without the RMT'S involvement and serve letters to conductors terminating their contracts and transfer them to the new on-board role.
Southern made a final offer, including a payment of £2,000, to all conductors once the dispute is settled.
The offer, which might be withdrawn after Thursday's deadline, was described as a bribe by the RMT.
Charles Horton, chief executive of Govia Thameslink Railway - which owns Southern, said: "We maintain this is a political dispute with the RMT hierarchy over their policy position, not a genuine trade dispute, otherwise the generous deal we've offered would have been accepted or, at the very least, put to members to vote on.
"We will consider all options to bring these unnecessary strikes to an end.
"Yes we want to talk - we have made it clear that we want to reach agreement with the RMT by Thursday lunchtime on the way we continue to implement our modernisation plans.
"But as we spelled out in our letter to the RMT yesterday, this can now only relate to the detail of our offer.
"We make no apologies about our social media campaign - its aim was to get the debate going and let people know how we are trying to modernise the train service for our passengers."
Liberal Democrat transport spokesperson Jenny Randerson said of Southern's deadline: "This is a very high risk strategy and the only likely outcome is that passengers will suffer yet again.
"This dispute has been allowed to drag on for far too long . Positions have become entrenched in both sides and the passengers are squeezed in the middle.
"We still have no positive plans or leadership from the Government, who should have stepped in months ago."
The union is planning 14 days of strikes over the coming months, starting with a three day walkout next week.