Read this nice article in High Scalability about scaling Social Networks.

Let’s say you have 200 friends. When you hit your Facebook account it has to go gather the status of all 200 of your friends at the same time so you can see what’s new for them. That means 200 requests need to go out simultaneously, the replies need to be merged together, other services need to be contacted to get more details, and all this needs to be munged together and sent through PHP and a web server so you see your Facebook page in a reasonable amount of time. Oh my.

Being a developer in eBay, I can comment on its architecture only.   Clearly eBay’s current scale can be attributed to sharding. Wherein a table is distributed in many databases.  Data belonging to a particular user reside in one of the databases only.

Now if eBay decides to implement social features in its site it will have to rearchitect its whole datamodel.

Lets say eBay developes a Twitter style Live status page for the user’s friends, then with the current architecture it will have to query the status of each friend from different database.

Some day when free I will write a post about how ebay can implement Social features.