It's very easy to to distinguish a 2 from the social-instinct. A person who is social-first is the way they are because they are aware of interpersonal dynamics and how we all relate to each other. As you know, a social-firstt person can be awkward or shy around people because they are so aware of that. What makes a 2 a 2 is simply their need feel loved by others. It's about being/seeing themselves as a lovable person.
Being Fe is not about being lovable (so that's what distinguishes it from a 2). It's closer to being a social-dominant person. I still don't have a grasp on how Fe is distinguished from the social instinct as well as I'd like (in fact, I don't have that strong of a grasp on any of these, but I am attempting to). Fe (for me) is best understood under socionics. But first, let's start with Fe. My friend is a 4w5 and how social dominance comes out is in her "relations" to others. She exists in reference to other people. She 'checks in' with others to know where she stands, what behaviors are okay with other or fits into the norm.
She also identifies with having Fe auxiliary (MBTI ...but socionics is easier), so if she truly is Fe, then I'd say Fe is about creating a "mood". In her case, it's usually a funny/silly mood where people can get along and interact positively with others. My 4w5 friend doesn't like it when she can feel negative energy. She likes to diffuse intense negative energy by either resolving a negative situation if she can.
The way I understand her "Fe' is that she is really communicative. She doesn't naturally have a "keep to yourself" energy, like I do. So the 4w5 and my other roommate like having someone to "come home to and talk to". For her, a happy home is a home where people are talking to each other and are actively creating an atmosphere where people can pleasantly talk to each other. However, I just want to go into my room not have to interact with others. It's not that I am not cordial and respectful. But for me, a happy home is a quiet home where I am not being nagged at to tell people how may day was or what I think about this topic.
And not to toot female-male stereotypes, but the easiest way I understand Fe vs. Fi is that Fe is like the overly-talkative wife (not the kind that tells you to clean up, but the one that wants to share your feelings/thoughts too much). Fi is like the closed-off husband that just wants to go to his den and drink a beer.
Edit:: I like using my 4w5 friend as an example, because it can be really hard when it's a enenagram 9. An enneagram 9 may look like they have Fe or social-instinct first when they do not. 9s are naturally other focused because they have a "diminished sense of self".