Unknown people on my Facebook friends list?

[Safat Islam]

From the information u gave i think there is nothing to be afraid of because as u said those people are known to you. Since they have deactivated their facebook accounts the links to their profile are shown black in color. And no one can plant virus on ur fb page it does not happen like that. What hackers can do is steal ur identity e.g. ur password. So, pls dont be afraid i hope there is no virus.
And u asked about stopping this from happening that is not possible any of your friends can deactivate their account and their profile link will turn black on ur friends list. Thats how it is.