The difficulty isn't finding a website that supports chat services, it's getting access to Selena Gomez in the first place that is going to be problematic.
Realistically, there are many people all around the world that would like a bit of Selena Gomez's time. Because her attention is in such high demand - even getting the opportunity to chat with her in private is extremely difficult.
That's not to say it's impossible though.
I guess the key to getting some alone time to chat with Selena is to decipher what it is you can offer that Selena actually wants.
She's got lots of fans, admirers, friends, etc... So you really have to think of something that is going to make you stand out... And, honestly, that's not going to be easy.
As well as being a talented actress and singer, Selena is also financially really successful. Perhaps if you could offer some sort of business incentive, then you might be able to make contact with Selena through her agency: CAA.
Alternatively, you can try to find out where Selena Gomez enjoys spending time hanging out, and then make contact with her that way.
Again, you'll need to work out a strategy so that, when you get a chance to speak to her, you can offer something of immediate value to her so that she'll want to continue engaging with you.
Maybe you could find out where she goes out to party, and then make friends with someone in her entourage. If you make enough of an impression, you might be able to rack her "inner circle" and become someone that she's happy to give out personal contact details to - you never know!