Facebook updates every two days I just bought a Nexus 5 (US version) and for the past 2 to3 weeks I have noticed that Facebook  updates every time I am connected to Wi-fi, I mean I am a developer so I can imagine that updates come out but not so often I assume. Should I just turn auto update off or is there something else that can be done?
Facebook does indeed update pretty often with random bug fixes as well as security, performance and stability updates. Often this depends on the company as well as the urgency of the update. Security updates tend to be urgent and are often shipped as quickly as the exploits are patched whilst minor bug fixes might be accumulated first and shipped as a larger update later on.

Automatic updates means less savvy users always have the latest version of the Facebook app and the relevant bug fixes and security patches.

If this bothers you, consider disabling automatic updates.

EDIT
This is the Facebook changelog taken from Changelog Droid