One of the secrets of Google’s financial success is its ability to match millions of small advertisers with millions of small content providers through its AdWords and AdSense services.

This strategy means that Google has financial arrangements with a very large number of debtors and creditors, a potential problem it simplifies by automating billing and payment. Unless advertisers choose to pre-pay, they are therefore charged only when their ads have been clicked by a sufficient number of people. AdSense affiliates, meanwhile, are paid only when their account is $100 or more in credit at the end of the month. Until the account gets to $100, they are not paid at all.

The problem with this model is that small content providers - hopeful bloggers with a few ads on their page - can wait a long time to get any money out of Google. Pootle along at a couple of thousand page views per month and it will likely take several years to clock up $100. These sites might seem worthless to a behemoth like Google, but of course the power of its business model lies in aggregation: it can bundle up huge numbers of tiny content providers to create a worthwhile editorial environment for any advertiser.

As I noted last year, Google wins both ways - it has no trouble running up enough ad impressions to get paid even by small advertisers, but can get away without paying the digital smallholders that actually furnish a big chunk of its playing field. If can often defer payment for years at a time, if not forever.

It’s not clear how many small ad-carrying sites will wither away without ever getting paid - but it is clear how much money is involved. Google has to record the total of these small debts on its balance sheet, under the label “accrued revenue share”. And this sum has been growing steadily ever since Google went public. At present, Google’s accounts show that it is hanging on to more than $370million - a float that it can of course invest in whatever way it likes to make more money.

When I blogged about this last year, some people argued that it would be a transient problem that had built up when Google first grew large, but that the amount would stabilise or even fall once most advertisers had clocked up a few years with AdSense. If that is indeed what will happen, the tipping point has clearly not been reached yet - just look at the graph to see the increasingly upward trend.

I think this situation is pretty unjust. There’s no reason why Google should set the cut-off at $100. It could comfortably lower the limit for pocket-money sites, now that it supports direct bank-to-bank transfers that run up costs of only a few cents per transaction.

As long as it’s the 800lb gorilla of online advertising, however, Google looks keen on keeping its growing hoard of other people’s cash.