Senators are not particularly familiar in the area of technology. As it is a hearing, Senators are free to ask queries they want to clarify. In this case, they do not understand how Facebook works and were clarifying that part.
Secondly, Senators do prepare for hearings behind the scenes, but it is not known if they were briefed for this one. But even if they do, they won’t be able to understand everything if they don’t use social media on a daily basis.
Committee members and staff usually plan extensively for hearings. Early planning activities commonly include collecting background information; preparing a preliminary hearing memorandum for the chair and members; discussing the scope of the hearing and the expected outcome; scheduling and providing public notice of a hearing; selecting witnesses; determining the order and format of their testimony; and preparing questions or talking points for committee members to use in questioning witnesses.
Next, when they eventually start to legislate social media – that's a different question. Senators get a lot of help when they write bills, so they would be able to make regulations even if they personally do not understand how social media works.