Who?
Who?
[Cueball is walking from left to right while Hairy follows him.] Hairy: ...I'm getting a ride with Katie to Adam's wedding. Hoping to see Brian on the way! Cueball: Oh, that's cool! Cueball: ...I can't keep living this lie, so I'm just gonna come out and admit it: I have no idea who any of the people you keep mentioning are.

Cueball and Hairy are walking while Hairy is talking about going to a wedding by sharing a ride. He names three people: the groom; a friend with whom he's sharing the ride; and another person he hopes to meet on the way, perhaps another guest at the wedding. The ellipsis at the beginning indicates he's been talking beforehand, and Cueball has listened to all of it.
Cueball at first replies with an "it's cool" sentence, apparently expressing interest, satisfaction or approval at the idea of Hairy meeting the people he mentioned. This usually happens when two people are talking about something they have in common, like meeting with friends at a social gathering.
However, Cueball suddenly expresses that he's been lying about knowing them, and he doesn't have any intentions of preserving such lie. Note that he didn't need to explicitly acknowledge to be those people's acquaintance, he might just have nodded or said expressions like "it's cool".
Part of a social need for inclusion, or as a way to continue a conversation, people sometimes agree with the person they are talking to, or feign knowledge of the people, things or topics that were mentioned.
The title text suddenly changes the situation by stating that it's possible Hairy has been talking to someone else using a Bluetooth earphone set . This hands-free device is used to communicate via phone call and is small in nature and only visible from one side of the face, so anyone who comes across someone using this device can at first wonder whether they're actually talking to them, because no phone can be seen. This situation could mean that Cueball has been hearing and / or talking to this person, who might not even be his acquaintance, given that he knows people that Cueball doesn't seem to know about, and that he might not actually be his friend. This is a hilarious exaggeration of people with some attention problems.
The title text is an allusion to 476: One-Sided , where Randall doesn't realize who the other person is talking to.
In 302: Names Cueball also has difficulty with names.
The problems with bluetooth headsets' inconspicuousness is a key point in 736: Cemetery .
Another example of not knowing someone is talking on the phone: [1]
[Cueball is walking from left to right while Hairy follows him.] Hairy: ...I'm getting a ride with Katie to Adam's wedding. Hoping to see Brian on the way! Cueball: Oh, that's cool! Cueball: ...I can't keep living this lie, so I'm just gonna come out and admit it: I have no idea who any of the people you keep mentioning are.
