[Idiom] to be into somebody

[beachboy]

I'm into Betty.
Do you think she's into you as well?

How common is it for native speakers to use the expression above? Is it dated?
Does it purely mean "to be interested in somebody"?
As for "I'm into Betty", can I say it either before or after starting a relationship?

 

[probus]

To be into someone means to be interested in that person in a sexual way. It originated in the hippie era of the late 60s and early 70s, and may be a bit dated, but it is still in common use, e.g. the recent chick-flick "He's just not that into you."

 

[J&K Tutoring]

I would take a half step back from what probus said and say it means the speaker is interested romantically (perhaps sexually, but not necessarily). We might use this same construction to talk about hobbies or other interests:

'I'm into stamp collecting.'
'She's really into Yankees baseball.'

As for "I'm into Betty", can I say it either before or after starting a relationship?

In my opinion, this works best before a relationship. After a relationship was established, it would be something more like, "I'm seeing/dating Betty."

 

[beachboy]

J&K, what if I'm talking to a friend, and he tells me: "Come on, Beachboy, you're dating Betty, but you don't really love her, do you?", could I tell him: "No, man, trust me. I'm into her."? Would it sound natural, although it happens after the relationship is established?
Anyway, would you say the expression "to be into somebody" is coming dated? I hear "to be into something" much more often.

 

[J&K Tutoring]

"Come on, Beachboy, you're dating Betty, but you don't really love her, do you?", could I tell him: "No, man, trust me. I'm into her."? Would it sound natural, although it happens after the relationship is established?

Yes, it sounds natural enough to me, but you're asking a 50-something American male to tell you if a colloquial expression is:

a. correct usage? Look up the word colloquial.

OR

b. outdated? How should I know how young Brazilians talk? I'm sure your friend would understand your meaning. If not, then he would ask for clarification.

Words and phrases have a tendency to get recycled and redefined in the process. What 'I'm into her' meant in my youth may or may not mean the same thing today.

 

[Skrej]

To be 'into X' is still widely used today, at least in the US. In general, today it means 'interested in X'. That interest may be of a sexual, romantic, or personal interest, depending on context. It often is preceded by the intensifier 'really'.

I'm into you. - I'm interested/intrigued/fascinated by you. I enjoy your company and presence.
I'm really into football. - I'm a great fan of football. I enjoy watching and/or playing it, and follow various teams/leagues.