Question: when did facebook start allowing everyone to join
[{'answer': '2006', 'provenance': [{'wikipedia_id': '7529378', 'title': 'Facebook', 'start_paragraph_id': 2, 'start_character': 335, 'end_paragraph_id': 2, 'end_character': 339, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]