A total of five men, who had stood against religious intolerance and at times criticised Pakistan's military, vanished within days of each other earlier this month, sparking fears of a government crackdown.
No group has claimed responsibility for their abduction and security sources have denied involvement.
But blasphemy accusations targeting the disappeared activists have been multiplying on Facebook and Twitter, triggering a flood of threats.
The allegation can be fatal in deeply conservative Muslim Pakistan, where at least 17 people remain on death row for blasphemy.
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"People are saying that Waqass is affiliated (to blasphemy). But the content attributed to Waqass on that page, he would never do that.
"Whatever content is on that page, we condemn it and he is not responsible for any of it.
"I'm just a student, I'm a simple person. I don't understand why are we being trapped like this," she said.
Rights groups have long criticised the colonial-era blasphemy legislation as a vehicle for personal vendettas as even unproven allegations can result in mob lynchings.
Several liberal online commentators have already closed their social media accounts in the wake of the accusations.
Pakistan is ranked among the world's most dangerous countries for journalists, and reporting critical of the military is considered a major red flag.
Rights groups say activists and journalists often find themselves caught between the country's security establishment and militant groups including the Taliban.
Faraz Haider, brother of Professor Salman Haider, who disappeared from capital Islamabad on January 6, said the charges were also putting the lives of other family members in danger.