In some cases, Google uses data sampling technology to reduce the load on its servers. Sampling is a way of shaping a representative sample in order to draw conclusions about the general population.
In other words, Google Analytics does not record 100% of page access sessions so the data may be inaccurate.
Additionally, Google Analytics and Facebook use different approaches to attributing clicks and conversions. For example, Google measures visits and Facebook measures clicks; which is technically not a one in the same indicator. Read more.
There is also the problem of data discrepancy when measuring mobile traffic if it comes from Facebook.