Social Circle Filler

As our plucky Ordinary High School Student comes into shot, he's seen to be the life and soul of the party of his circle of friends. However sighting a girl who he's never seen before, he makes his excuses with a quick joke and he says he will catch them later. As he goes to see who this girl is...

Only he won't.

An hour and a half later. He's knee deep in the Big Bad's plot and claiming that no one can possibly help him or understand. Hang on a second says the audience. Wouldn't his friends want to know about this life altering experience? Or help?

A Conservation of Detail so not to make out that our protagonist is a loner. An extra or two is roped in at the start to flesh out our hero's social circle, but at the same time they are not required later as the character is drawn into their new environment. But their absence is noticeable.

This trope is specifically applied to when an extra or Living Prop is implied to have a friendship or relationship with a main character. See also Forgotten Fallen Friend. Compare Friendless Background and related to Twenty-Four-Hour Party People, where characters have an ever-shifting group of friends who only show up when they have a party.

Examples of Social Circle Filler include:

Anime and Manga

Naru/Molly from Sailor Moon. Usagi/Serena's best friend until she meets Ami and Rei, then vanishes from relevancy soon after as more and more Senshi are introduced. An even better example are two token female classmates who talk with her for a few episodes until the actual main Supporting Cast is introduced—at least Naru showed up again.
Psyren In the beginning, Ageha is seen with two friends in class, which he seems to be on good terms with. Once he gets roped in with Amamiya into the world of Psyren, we pretty much never see them again. Instead, Amamiya and Hiryu become his closest friends.
Tokyo Mew Mew: Ichigo's friends Moe and Miwa appear in the first episode, and then once or twice afterwards, before vanishing completely.

Film

10 Things I Hate About You: Patrick is seen at the start of the film to hang around with a bald biker guy, who he smokes with and is his lab partner. As Patrick gets further involved with Kat, the character disappears altogether. When Patrick needs anyone to talk to later on, he goes to Cameron.
Spider-Man: Mary Jane Watson is flanked by a Girl Posse for the first half of film one. However they quickly disappear and do not reappear for the rest of that film and despite seemingly being quite close, do not appear at her wedding for the second movie.
Early in I Love You, Man we see Peter has several female co-workers who he seems to get on perfectly fine with; strangely, they are never mentioned again even though the plot revolves around Peter's lack of a close (male) friend to act as best man. The possibility of one of his female friends from work acting as his best man is never raised, even as an option to be rejected.
Sam Witwicky has a best friend for the scene where he tries to go to a party, early in the first Transformers movie. The only other time we see him is when he blows Sam off when he calls to tell him his car is alive.
Bennet from The Sorcerers Apprentice.
Tim. John Connors friend from the begininng of Terminator 2: Judgement day
Ray from Cop and a Half, also used to contrast the protagonist's strained home life.

Live Action TV

How I Met Your Mother: Robin in the very first episode is shown to laughing and joking with a few girls who are presumably her friends or co-workers. Once Ted is in her life and she joins the main cast, no mention of who they were is ever given.
Buffy the Vampire Slayer: Cordelia's fellow cheerleaders or Oz's band.
Scrubs: Dr Cox is implied to actually have a circle of friends outside of the Hospital in season one. Shown in one episode, they are not used again and later episodes establish him as friendless and lonely.
Friends: Whenever the friends cast threw a New years party or a baby shower, lots of unknown friends would suddenly be shown. But never the same people twice.

Literature

Kayla is this in the House of Night series. She appears briefly in the beginning of the first book and briefly again near the end. After that she's pretty much forgotten in favour of the Five-Man Band.
Justified in Brandon Mulls novel The Beyonders : A World Without Heroes, which starts off with a lot of interaction between main character Jason and his friends, the girl he likes, etc. They all become pretty irrelevant once he falls through into another world and discovers he has no way back, or even to send a message. His friends and families reaction to his disappearance is unknown, although Jason does think about it. Since Jason ends up back on Earth at the end, we'll have to see if the trope ends up being averted in the sequel.

Visual Novels

In Yume Miru Kusuri, Takeshi and Misaki are the main character Kohei's better friends. But once one of the three routes have been picked, they are completely forgotten.