There are two things worth mentioning here.
Firstly, this is not true for all atheists; namely that they are all ex-religious people, as the meme seems to suggest. Also, in my experience I have found that many atheists are only atheists because of feelings, or emotional reasoning as opposed to intellectual based reasoning. You will be surprised at just how many people are only atheists because of the unfortunate loss of loved ones, or because of diseases & illnesses etc. Other atheists simply cannot stand the biblical God who stands as the ultimate authority while many others view Christianity as repressive, or they just can’t stand the way the church sees certain things (such as pre-marital sex, or homosexuality, for example). These atheists are only atheists because of feelings. It is, however, true that many of them are ex-believers and we needn’t deny that. I also don’t at all suggest that all atheists base their reasoning solely on emotions because that would be false (although there is always emotional attachment involved in all people irrespective of worldviews). However, it is also true that there are many Christians who are ex-atheists. So the atheist ought not to somehow think that he has a monopoly on conversions or that it’s only ever believers who end up atheists. The fact is that the door swings both ways.
The meme then somehow wishes to imply that this makes the atheist’s position more credible, especially since it reads “I already know about religion. That’s why I’m an atheist.” Firstly, I’d challenge any atheist claiming such a thing for no atheist has a complete understanding of world religions. The atheist in this meme is clearly making a claim to knowledge. Most atheists might know some things about Christianity and a thing or two about Islam but their knowledge, more often than not, is extremely limited beyond that (judging by some atheists it also would seem that they do not even possess a basic understanding of Christian theology, let alone the theologies of many “religions”). In other words, the atheist is claiming a level of knowledge beyond his epistemic capability. In truth the meme’s message would have been much more credible should it read something like “I already know about Christianity. That’s why I’m an atheist.” Then we could engage in debate that focuses on Christian truth claims. That would be more to the point.