Preservation is helping the Lord Ruler preserve things would be my guess. With the Lord Ruler in complete control, Preservation is basically given free reign to preserve everything it can, thus leading to the stagnation of society others have mentioned.
Also, perhaps it is not Mistborn who are of Ruin, but humans in general. Humans were around far before the Ascension, and so could have been created by Ruin long before he was imprisoned.
I forgot about that quote, Reaves, and now that I think about it, it might have just been more of what went on before Vin freed Ruin. Nobody went to the Well though, so Ruin slowly started seeping around the edges, changing metalminds and sending the deepness upon the world. Without a Hero of Ages to take the power in the Well and use it themselves, Ruin could possibly tap slightly into it and use it to weaken his prison. Then Rashek went to the Well, took the power, re-imprisoned Ruin as strong as ever, and sat back to wait for the next time he would need to go to the Well of Ascension.