I don't think that they crossed the borders after the FB post. They have been crossing for a long time and settled in and around WB by our beloved Momta didi for purposes, she knows best.
It is just that they forget that they are in another country as illegals and readily take up arms and come in droves when they see something on Facebook.
You are right, to improve law and order in West Bengal we need to seal the border.
So here responsibility lies with BSF and center needs to be strict.
Just to mention here from SCR border security act
The Ministry stated that as on date, the Central Government has given powers to the BSF personnel in border areas under the Customs Act, the Passport Act, the NDPS Act and the CrPC by way of issuing suitable notifications under the respective legislations. These powers, so conferred, primarily relate to arrest, search and seizure within the prescribed border belt, which is 80 Kms. in the State of Gujarat, 50 Kms. in the State of Rajasthan and
15 Kms. in the States of West Bengal, Assam & Punjab. No such limit has been prescribed in the case of J&K and five North Eastern States viz., Meghalaya, Nagaland, Mizoram, Tripura and Manipur.
BSF is equally responsible for not able to stop the riot as with West Bengal police.