Ungefähr has approximately three times more entries than circa in a corpus (Wortschatz-Portal Uni Leipzig).
Circa, zirka and ca. together indeed have more than three times as much entries than ungefähr in a corpus (Wortschatz-Portal Uni Leipzig).
That said, the use of circa is restricted to numbers though. I can't think of any example where it wouldn't sound off, when not referring to any numbers. An example where its usage is incorrect.
*Das ist circa mein Problem.
In this example, in contrast, the word ungefähr fits perfectly.
Other examples for ungefähr:
Das kommt nicht von ungefähr.
Das ist ungefähr das gleiche.
Sie waren ungefähr gleich begabt.
Eine ungefähre Angabe würde genügen.
In any of these examples circa doesn't sound well. However, when referring to a number again, you can take circa even if this number is not mentioned.
Es kamen circa gleich viele Leute.
Er ist circa gleich groß.
Still, I'd tend to use ungefähr in those sentences though.
As a conclusion: the claim that circa is used more often than ungefähr is true. You should remember that circa is restricted to numbers. As ungefähr has a broader area of usages and is still less frequent, I think circa is indeed almost solely used for numbers.
This is in contrast to my original assumption that usages are equally frequent. The corpus proved me wrong on this. Still, this outcome surprises me. I'd be interested into data for colloquial only; unfortunately, I don't know any sources for that.