regarding who is paying bitcoins
bitcoins are rewarded by yourself when you've successfully solve a block (es explained
here) and whole network agrees to it. currently there are over 6 million bitcoins in existence out of 21million in total, why 21m - have no idea yet.
system is made way that each block to be solved approximately every 10 minutes (6/hr), if hashing rate grows with more users on board blocks will be solved faster, for this to counter balance speedy block solving there is a difficulty variable which tries to keep rate of 6 blocks in hour, every 2016 blocks difficulty is reset (recalculated) to reflect current hashing rate, this works out that on average difficulty resets every 14 days, in reality past few difficulty increases been happening every 10 days due to ever increasing number of new miners and hardware coming on board all competing to solve a block for a reward of 50BTC, every 210000 or so blocks reward is set to be 1/2 less, which works out to be close to every 4 years reward is going to be 50, 25, 12.5 until somewhere in 2130 system will stop rewards, but to keep network secure miners will be getting transaction fees of transactions included in solved blocks by them.