It is easier to fall out of love than in love.

Depends on whether the falling out of love is voluntary or involuntary (i.e. the love might not have been mutual. then 'falling out' is tough ...).

I am fairly sure statistically people fall in love more times than they fall out of love.

For every time someone falls out of love, they must have first fallen in love, and some loves never get fallen out of. Which makes it seem easier to fall in than out.

I can't think of any other metric to use than frequency.