Stop me if you’ve heard this one before. Powerful more-or-less user-run news aggregator Digg.com is apparently for sale.  But unlike the last handful of times you heard that, this time four potential buyers have been identified, and are performing due diligence towards an actual sale. Perhaps you’ve heard of two of them: Microsoft and Google.Digg has been working with investment bank Allen & Co, and is pitching big tech and media companies on a sale. It is even prepared to take less than the $300 million