You should read the original, not the craptastic blog summary. It does indeed mention the usability differences between the two sites, then goes on to discuss

You should read the original, not the craptastic blog summary. It does indeed mention the usability differences between the two sites, then goes on to discuss how the prime deciding factor between a teenager using one site or the other is rather: "Simply put, they go where their friends are." Further it's not ascribing racist motivations to anyone's choice of social network, instead making the observation that, by going where their friends are, existing socio-economic divides are being perpetuated into social networks.