Twitter has opened up its self-serve ad platform to another 10,000 marketers as it ramps up efforts to monetise its service.

Self-serve ads launched last year but initially only 100 companies were involved in the test phase.

The latest roll out is in partnership with American Express, so only Amex cardholders or merchants who accept Amex can sign-up.

In return Amex will give each buyer a $100 credit to sweeten the deal.

While the number of places available on the platform is limited for now, Twitter told AllThingsD that it will let in more buyers throughout the year in further groups of 10,000.

It shows that Twitter is serious about making money from advertising but is retaining a cautious approach, presumably so it can modify and develop the platform as it lets in more users.

The self-serve approach has worked well for Google and Facebook, so it is no surprise to see Twitter following suit, however it will have to overcome doubts about how effective its platform is.

Last year we reported on claims that marketers were dubious about the ROI on ad campaigns using the social network.

So we can probably expect to see Twitter using data from this limited test phase as case studies for how effective truly can be.