The question is often posed to atheists on social media, "Why do you care about what we believe in? Why don't you just keep your non-belief to yourself?" There are several different ways to answer this, so I'm going to try to summarize at least some of the reasons for our being so 'militant' about what others choose to believe.
1. Atheist commentary on religious beliefs often is triggered when believers post their beliefs on social media. Thus, a big fraction of the commentary by atheists is driven by initiating commentary by believers. It's a reaction to what we may see as illogical nonsense, or something contradicted by evidence, or simply bizarre. When such commentary vanishes from the social media, so will a lot of atheist counter-commentary.
2. We live in a time when thinking based on logic and evidence is more important than ever. It is, in fact, dangerous when believers are so intensely driven by their religious dogma, they're willing to kill and even die in the process to further their beliefs. It wasn't atheists who crashed planes into the World Trade Center! Much of the existing armed conflict in the world is (and has been) associated with religious, sectarian clashing. Many of us think that anyone who can influence the fate of our entire world should not be someone who looks forward to an afterlife of bliss. Given the scientific issues that confront our nation - global climate change, stem cell research, genetic engineering, pollution of air and water, hazardous weather, and so on - we need more rationality, not less. Our nation seems to be on an anti-intellectual, anti-science roll these days, which some of us see as potentially very dangerous.
3. Many believers are "moderates", some of whom may not even support "witnessing" and proselytizing via social media, and certainly have no wish to participate in acts of fanaticism to seek the advance of their beliefs. They often seem to be upset when atheists post something that's not a response to some religious post. Are they similarly upset when religious believers post something that's not a response to some atheist post? Even if they are, which seems unlikely, don't atheists have the same right to offer their thoughts about religious topics? If you don't like our comments, just ignore them - after all, that's what you're asking us to do!
4. Even when not responding to something a believer posted, atheists use social media to share their thoughts with other atheists, perhaps offering a new argument against religious belief, or seeking to stimulate discussion on some religious topic. And there are a lot of doubters in the religious world - people who are beginning to question the dogma they've been indoctrinated with for years. It can help them to feel less isolated from others to realize that others have similar questions and doubts. Even more so for the 'closet atheists', who have arrived at atheism on their own, but fear that disclosure of their atheism will cause an overwhelmingly negative reaction from their family and friends. If an atheist is unafraid to post commentary about religious beliefs, that commentary can be of value to those unable or unwilling to 'come out' about their atheism, allowing them to know there are others out in the world who share their doubts about religion.
5. Religious beliefs in the USA have made considerable inroads into the way our nation is run. Many of the laws on what defines criminal sexual behavior have their origins in the morality of religious believers. We have 'blue laws' of various sorts that are directly attributable to the dominant christian religion in this nation. Many politicians, even those not religious zealots, have found it beneficial to wear religion on their sleeves, and to support things like public school prayer and the slogans in our pledge of allegiance and on our currency. The christians claim we atheists are waging war on their religious freedom - but the reality is that we simply are opposing their efforts to push their beliefs on everyone in this nation. Being in a distinct minority means we often fail to stop the creeping theocracy we see going on. Our only protection is the Bill of Rights in the U.S. Constitution, and many believers would like to see our nation become openly theocratic - a christian nation in the same sense that muslims now run theocracies in some nations. We atheists don't share much other than the absence of belief in a deity, but most atheists are fearful for the loss of their constitutionally-guaranteed freedom of non-belief. That alone justifies our 'militancy'.
I have no wish to 'convert' anyone, but I certainly would like to see a more consistent use of logic and evidence to base decisions that affect everyone. If you don't support the use of contraceptives, then don't use them! If you don't support abortion, then don't have one! If you believe certain sexual acts are sins, then don't do them! If you don't believe in evolution, then you can teach your kids your views at home, not in public school science classrooms! If you don't believe in same-sex marrage, then don't marry someone of the same sex! You're entitled to your opinions and beliefs - just don't expect everyone to share them and don't support laws and actions that restrict the choices of others solely on the basis of your religious beliefs. When we have a society that unambiguously and rigidly supports the separation of church and state, then you'll be seeing a lot less atheist commentary. Until then, get used to it!