Guess I was up against the person who wrote this post https://leetcode.com/discuss/interview-experience/1338185/Facebook-London-Interview-Experience

Screening Round

1. Given a list of strings, return whether the list is sorted or not
2. Find if there exists a subarray with sum k

Got call for next rounds within like 2 days.

Round 1, System Design
Proximity Server
After explaining the top level design, was asked to straightaway jump to the core algorithm part. Couldn't land up on geo-hashing even though I had seen that in my prep. Proposed a naive soln to get nearby co-ordiantes. Guess I messed up this round the most.

Round 2, Coding

1. Given 2 linked list, which might intersect. Return the first common node. went with hashing.
2. BST iterator. went with the stack approach.

Very interactive round, discussed various solutions after coding my approach. Very friendly interviewer.

Round 3, Coding

1. Sorted rows in a 2d matrix, return the first col where you can find a non zero val in at least one row. proposed a linear soln, where you start from the top right corner of the matrix. could as well do a binary search on the possible answer. but didn't get into that.
2. Given a list of strings, consider them to be sorted according to a custom alphabetical order. Return a possible alphabet ordering that holds good in the input list. went with topological sorting.

Again, very interactive and friendly. went smooth.

Round 4, Behavioral
Can't really assess how this went. questions on challenging projects, problems with colleagues, feedback received from peers/managers, managing priorities etc.

Got reject after a week. Good and smooth interview experience, but not the desired result obviously. friendly recruiters.