Facebook or Twitter, Which is more effective?
How shall we measure the effectiveness?
What do you want to achieve?
Speaking from a pure personal perspective, in terms of getting links back to your website / webstore I would go for twitter but even so twitter or gab requires you to have content to share with the public and it works more or less like press release. Facebook and twitter really do not compare 1:1.
Always make sure what you have original content on your own "domain". Facebook or twitter make take your postings down or limit the exposure and then your content is lost.
Like exelorien is saying, what do you want to achieve exactly ? Is it traffic to your website ? Well I would not depend solely on either of them. Facebook actually severly limits exposure (organic that is) back to your website unless you advertise or have a lot of true fans / followers on facebook.