I'm not sure how I feel about "Pay with a Tweet", but at least people can actually customize the message to reflect what they thought of the service.
I would rephrase the "Already liked us?" to "Skip this step" or something like that.
Are you okay with requiring someone to shout "I like this service" to be able to use said service, on first use? Does it feel right to disclose the like requirement after the user has already put time in using the service?
It sounds backwards to me at least and you're definitely alienating some people, but if you think it's no big deal then you're entitled to your own opinion.
You don't need an A/B test to tell you that "If you enjoyed our service, please Like us on Facebook." is a friendlier, more positive request. People have already tweeted about it more than they liked it on Facebook without additional prodding. Let the site stand on its own merits, rather than trying to force promotion through potentially sleazy methods.