Estimated target audience - large fluctuation?

Same add posted twice with big difference in estimated audience

[bemused]

First off, I'm new to facebook and new on this forum, so please forgive any errors of protocol on my part!

I posted an add on facebook and was delighted to discover how the estimated audience size was presented as I refined my target audience. On my first run the audience was estimated as 1,480. A few days later I ran the ad again, with the same targeting criteria, and the estimated audience was showing as 940.

I targeted a specific age group, at secondary school, with just one "like", UK wide.

I find it difficult to believe that so many people "fell out" of the audience in such a short space of time, so can someone help me understand what might be going on here?

Grateful for any thoughts.

[weeks]

In a small, targeted audience like that, with kids and parents, it is tough to say. As the kids age, their habits change rapidly. They are a moving target. They'll all run to this, then run away from it.

Add to that, FB is in flux right now. There are a lot of newbies coming on and leaving as they're not making it a habit. You are wise to watch the numbers carefully. We're going to see a lot of swings in both directions, up and down. (Has Facebook peaked? You have provided one datapoint that says yes, or at least maybe...)