1- How filehost site make money?:By selling premiums
2- Why they pay people to upload to their sites? So that more people upload for them,hence more premium sold and more money for them.Basically an incentive to uploaders to upload for them
3- Why so many new Sites come these days?Maybe because they think they can make lots of money quickly and/or become the next rapidshare,hotfile
4- how can we trust a filehost site when all are so near in options.: go for the more established ones,google out,ask fellow uploaders
5- it is better to make filehost sites and use yourself or better work with others sites. You need lots of money to start a filehost site+legal team blah blah blah, so better to go with established ones

Hope i answered all your questions
Regards