Is it permissible to add non-mahram women to his list of Facebook friends for the purpose of da‘wah and offering advice?

Firstly: 

Facebook has pros and cons. The benefits of Facebook and the
harm it may cause are connected to the reasons why one signed up for it and
how one uses it. 

We have discussed this website in the answer to question no.
137243; please refer to it. 

Secondly: 

We do not think it is permissible for a man to add any women
who are not his mahrams to his list of friends, and especially we do not
think it is permissible to correspond with them and chat with them. What is
more dangerous is when you can see them. That is because this opens the door
to fitnah for those who get involved in it, and the tragic consequences that
result from forming relationships between men and women are too many to list
and too well-known to need mentioning. The Muslim should not be deceived by
the Shaytaan’s making these relationships attractive by claiming that they
are for the purpose of da‘wah, exhortation, advice and helping others. If a
man really is keen to call people to Islam, there are millions of his fellow
males who need that, so he should hasten to add them and help them. The same
might be said to sisters who want to help others: they have to do that their
fellow females and leave calling and advising men to other men. 

We have discussed the ruling on correspondence and chat
between the sexes in several fatwas. See the answers to questions no.
78375,
26890 and
82702. 

In particular, see the answer to question no.
98107, where we mentioned ways
in which the Shaytaan causes daa‘iyahs to fall prey to the fitnah of women
on the internet. 

What we hope is that our brother will be sincere to himself
and avoid all doubts and close the door to fitnah; he should refrain from
adding any non-mahram women to his friends list. If he has done that
previously, he should hasten to remove their names from his list (“unfriend”
them). It is better for him and for them. And we ask Allah to protect the
Muslims in their religious commitment and protect them against the
temptation of women. 

And Allah knows best.