Keep a counter of friends for each user. Most users will have zero friends, no need to look them up. Also, if that's such a bottleneck, create a separate webse

Keep a counter of friends for each user. Most users will have zero friends, no need to look them up. Also, if that's such a bottleneck, create a separate webserver in C++ and keep friend relationships in RAM in some sort of an efficient data structure, something like Map [UID]->[set of friend UIDs]. Queries should be lightning fast (even if you pass a thousand UIDs in one query).