How do I use yahoo & Facebook with my mobile?

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What if someone doesn't have a computer or is living somewhere for a vary long time without his/her computer.
If he/she wants to access his/her Facebook account & Yahoo account with his/her mobile, What should he/she do?
I mean how do I use yahoo & Facebook with my mobile?
I don't know how to do that & I don't know any trust worthy people who can help me (everyone I know is evil, pure evil).
And I wanted to know an other thing:
Suppose I'm using my mobile for accessing my yahoo & Facebook,so if someone enters my yahoo ID or Facebook ID with their computer will I be able to know that(like a massage arrives to my mobile/yahoo/ telling me that )?
Please I need to know these answers so please answer m & don't bother answering my question if you don't know the answers because I don't want any silly answers.
Oh please help me!!!