We present LinkSO, a dataset for learning to rank similar questions on Stack Overflow. Stack Overflow contains a massive amount of crowd-sourced question links of high quality, which provides a great opportunity for evaluating retrieval algorithms for community-based question answer (cQA) archives and for learning to rank such archives. However, due to the existence of missing links, one question is whether question links can be readily used as the relevance judgment for evaluation. We study this question by measuring the closeness between question links and the relevance judgment, and we find their agreement rates range from 80% to 88%. We conduct an empirical study on the performance of existing work on LinkSO. While existing work focuses on non-learning approaches, our study results reveal that learning-based approaches has great potential to further improve the retrieval performance.