'it' to identify people

When we are trying to identify people, we normally use it to refer to them, so in your example, Fabiola, the answer would be: It’s Susan, not She’s Susan.

Incidentally, for the question, we would probably use the definite article and say: Who’s the girl in the blue T-shirt? as we are talking about a particular girl and a particular T-shirt.

Here are some more examples of it used to identify people:

• Is that Prince Harry talking to the TV reporter? ~ No, it isn’t. It’s Prince William.
• We must go. Let’s get the bill. Is that our waitress over there? ~ No, it isn't. Our waitress has got longer hair.

However, in a face-to-face situation which is more personal, we sometimes use the second person pronoun instead of the third person.

Compare the following:

• Hello! We’ve met before, haven’t we? It’s Tom, isn’t it? ~ Yes, it is.
• Hello! We’ve met before, haven’t we? You’re Tom, aren’t you? ~ Yes, I am.
• Hello! My name’s Susan and you are...? ~ I’m Rachel.
  

'it' to introduce clauses

It is a very versatile pronoun and can be used in many different ways.

It is used with adjectives to introduce a to + infinitive expression:

• It was kind of you to phone. (NOT: To phone was kind of you.)
• It’s always nice to talk to you. (OK, BUT MUCH MORE FORMAL: To talk to you is always nice.)
• It’s good to hear you’re feeling better. (NOT: To hear you’re feeling better is good.)

It is also used to introduce a for + infinitive construction:

• Talk to him. It’s important for him to understand that she won’t always be there.
• It will be easier for you to study if you get rid of that boyfriend.
  

'it' as subject instead of that or wh- clause

Instead of using a that-clause or wh-clause as the subject of the sentence, we often prefer to introduce it by using the pattern with it + be + adjective/noun and follow this with the clause.

Compare the following (the examples in brackets () are too awkward with this word order):

• It’s a pity (that) you can’t attend the meeting. (NOT: That you can’t attend the meeting is a pity.)
• It’s not clear why he resigned. (OK, BUT MORE FORMAL: Why he resigned is not clear.)
• It’s very important that he should be told about this immediately. (NOT: That he should be told about this immediately is very important.)

it + verb + object + clause.

Compare the following:

• It worries me when he drives so fast. (NOT: When he drives so fast worries me.)
• It surprises me that he hasn’t been caught. (NOT: That he hasn’t been caught surprises me.)
• It would please me if he got a speeding ticket. (NOT: If he got a speeding ticket would please me.)

Finally, with the verb take, when we are talking about the time needed to make things happen, we normally use an it-construction:

• How long does it take (you) to get to work? ~ It takes about ten minutes by bike, twenty minutes by car.
• Would you mind translating this letter for me? ~ No, not at all. It won’t take (me) very long.