Unfortunately, people fake friendships to gain something; usually social status, money, love, acceptance...for literally every reason you can think of. Not every person is like that, but there are friendship poachers (they are friendship fakers for sure), and social status climbers (they also are friendship fakers for sure).

Humans are social animals who will use and abuse others to get their emotional, psychological and physical needs met. Unfortunately, the few who have solid characters easily fall prey to the friendship fakers because they are good people.

Signs of a fake friendship:

1. Their friendship is conditional.
2. Your friend acts differently around you when you are in another social group.
3. Your friend gossips about you behind your back, then denies it to you if you call them out on it.
4. Your friend will cancel plans with you and not apologize or even appear to care that they hurt your feelings, at the last minute, if something else better (in their view) comes up.
5. If they have an argument or disagreement with you, they use that as the impetus to end their friendship with you (real friends don't part ways over disagreements, they work through those with each other).
6. Your friend is a mooch and always wants or needs something from you.
7. If you say "no" to them, they'll drop you like a hot potato and stop communicating with you.
8. They sabotage your goals and try to undermine your self confidence.
9. They are always negative and complain a lot around you.
10. They gossip about others to you and expect you to take their side always.
11. They are horrible listeners. They don't really care what you have to say and they don't really listen to your opinion or validate it.
12. Fake friends treat your reasonable expectations for a friendship as a nuisance and put you down for having boundaries with them, because their agenda is to manipulate you.

I'm sure there are more red flags but these are ones I experienced first hand (and still do, if I'm not careful with people in real life or online).