Facebook is keen for retailers to link up their customer data with its own social data on users, and eBay has become one of the first to step up.

Most retailers are very cautious. The idea of taking information off people’s Facebook profiles and using it to sell them things makes many companies hesitate – the potential privacy backlash could bring bad PR, and retailers are beholden to customers’ sense of ownership over their Facebook pages.

There’s nothing to stop it happening, though. Facebook users get a free service, and they pay for it by providing information about themselves. It might not feel like that to the average user, but slowly the data they give to Facebook will be used to help shape marketing and merchandising strategies. If they don’t like it, the only real answer is to leave Facebook.

That is of course perfectly possible – other networks, like MySpace, have risen and fallen with surprising speed. Facebook and its retail partners will have to tread carefully.

But there are plenty of opportunities to use Facebook and its wealth of information on people. Ebay’s approach is a plan to release a range of ecommerce applications with social networking capabilities, and services that are shaped by personal information.

Its X.commerce project opens up its ecosystem to outside developers, who, it hopes, will play a role in developing a new raft of ecommerce and m-commerce offers. As well as announcing the X.commerce project this week, it also announced its intention to work with Facebook to make online shopping more personalised.

Facebook’s Open Graph – the footprint of connections that Facebook users produce as they interact with friends and online content – will be integrated into applications eBay develops.

It’s not clear yet what the partnership will produce, but eBay is one of the first retailers choosing to push ahead with heavy use of Facebook data.