AlyssaS - 11:20 pm on Feb 6, 2011 (gmt 0)
Re the 10 month expiration, that seems too short IMO. People buy domains for a year, and most hosting is annual, so it seems reasonable that a more accurate figure would be in the 24 month range. The thought being, they go online, give it a year, it's not working so they try one more year, it's still not working so they pull the plug.
My reasoning was this: spammers look for dead blogs and forums to post their links on (sites that are maintained simply zap the spam). So, they are dropping links on sites that are perhaps due to expire - people don't bother to renew domains that they arn't maintaining. So - the links will always have been created in the ten months or so before expiry. As the old set of links drops off, because the domains arn't renewed, they are creating fresh links on domains still with a bit of time to expiry, so they sort of maintain their position in the SERPs because the total number of links they have stays the same.
But if you placed a filter on the age of the links, they would end up with zero, as all these short term links would not be counted.
Am I being too simplistic?