I'll let you know
What it really means
Sometimes genuine, but often a soft way to end the topic without committing. After job interviews, "we'll let you know" with no timescale is sometimes a gentle no. Silence is usually the answer.
Context, tips & common mistakes
In social invitations, "I'll let you know" is most often a soft no that spares both sides an awkward moment — the notification rarely comes, and mild follow-through is not expected. Decode it by counting: one chase-up ("still thinking about Friday?") is fine; after that, take silence as the answer it politely is.
In professional contexts it is more literal — a recruiter or landlord saying it usually will let you know — but a timescale question ("roughly when should I expect to hear?") is acceptable and clarifying. Using it yourself works both ways: it is a kind way to avoid an on-the-spot refusal, but leaving someone hanging before a real event with numbers and bookings crosses from polite to inconsiderate, so convert it into an actual answer within a day or two.