Yes it can, with the Top Referrers Card.
The Top Referrers Card appears alongside any search or saved segment.
This Card highlights the top referrer, plus a number of the next most common referrers, and indicates how many users visited you via that referrer. If you'd like to quickly zoom into the users from a single referrer, just click one of the bars in the Card and your search is instantly created.
Why does the referrer sometimes show as "unknown"?
A referrer is a web page that sends visitors to your site. If a user navigates directly to your site, then no referral information will be available for FullStory to include.