Data portability--the capability to transfer one's data from one platform to another--has been described as an important tool for giving individuals more control over their data. It is defined in significant regulations such as the GDPR, and implemented by major online platforms. Unfortunately, there is a lack of research investigating internet users' perceptions and expectations of this technology in specific contexts, which is vital for building effective designs. One particularly important context for studying user perspectives is social media, since it is deeply embedded into daily life and is particularly complex regarding the value and portability of user data. This paper addresses that gap through a survey and interviews of social media users in the United States. We identify current attitudes and practices toward controlling their social media data, and examine participants' impressions about the extent to which data portability may enhance their control. Participants had generally favorable impressions, but had differing opinions about what forms of control are important and the extent to which those could be served by data portability. Based on the results, we propose future directions for improving users' control in the context of social media, such as fine-tuned filtering of data to be transferred and ways to coordinate transfers alongside social contacts.