There is also a simply and rude economic reason which push the business in huge new girls turnover, it is that to avoid the girls to become the new supermodels, means that the tariff stay under control.
So i also think that it is an important reason why clients prefer to often renew the girls.
So automatically, this gives to agency a big need to have new things to propose all the time, but that need has always been there, it has just changed a bit in the last years.
The new faces blogging thing is a funny things, nowadays, it is clearly laughable. It started with few bloggers with good eyes, spending time to spot good girls. Now they all become lazy, they receive girls proposal in their mailbox sent usually by the worst mother agents for who blogs is the only way to be visible, so the level got down the pants. Except models.com who is still rather selective about what they feature (and who are anyway the only site really regularly checked by the industry) i hardly read any other blog nowadays.