Ambisocial
(noun) A person that feels no difference between being with other people and alone.

(adjective) Being indifferent to socializing with others.

They either never feel alone (positive variant), or always feel alone (negative) regardless of whomever is present. Positive ambisocialism normally arises from having grown up in constant social contact. And likewise negative ambisocial individuals have grown up spending little to no time with others.

An ambisocial differs from an antisocial in that social scenarios do not drive negative behavior or change a person's behavior at all.
Ambisocials also differ from an asocial in that the prospect of socializing does not impact their decision of choosing to go to a social event or being alone. Ambisocials will neither avoid nor gravitate towards social events.

To an ambisocial, the vibration of their phone means nothing. Either 500 new notifications, or zero, it's all the same to them.