Equivalent word for 'Unfriend' on Social media sites?

I understand that 'Unfriend' is not a valid English word. But, is there an equivalent word for the act of 'Unfriending' someone, like on Facebook?

'Not being on talking terms' is something a friend suggested. I am looking for one word here.

from @Hugo :
Because social media "friends" are sometimes just acquaintances and not necessarily actual friends (and sometimes not even acquaintances), a good answer will also apply to this weaker relationship.

Estrange is one possibility, and depending on circumstances alienate is another.

(Alienate doesn't seem to me to be appropriate to social media, but that aspect of the question was added after my original response.)

Unfriend is most certainly a word. The OED has this citation from 1659:

I Hope, Sir, that we are not mutually Un-friended by this Difference which hath happened betwixt us.

There is also an entry for unfriend as a noun, meaning, unsurprisingly, ‘one who is not a friend’.

There can be no denying that unfriend is a word even in the absence of any such historical evidence. Anyone who uses Facebook knows exactly what it means, and I see no reason for a substitute.

We also have, incidentally, that useful word uninvite, first found in Pepys's Diary in 1665.

While "unfriend" it not in and of itself a valid English word, it is, in most contexts, understood to be a construction of "un–" and "friend".

Using "un–" as a negation is a quite common idiom, ("unfriendly", "unstable") and there is an argument to be had that it is valid in the general case for words with no specific antithetical form.

Edit:

An alternative in the same genre is "defriend", which is also not a valid English word, but can be constructed using a similar mechanism, with "de–" rather than "un–", as in "destabilize", "devalue".

Both alternatives are easily understood and pithy, but neither is an actual English word.