This problem has been considered by a few authors. Versions for $a$ and $b$ *rational* were considered by Hooley and Guo (independently). See 

Hooley - On ternary quadratic forms that represent zero.

Guo - On solvability of ternary quadratic forms.

Hooley obtained the (sharp) lower bound of the form $H^3/(\log H)^{3/2}$ for the corresponding counting problem. Guo obtained an asymptotic formula for the slightly different problem where $a$ and $b$ are assumed to be square-free (i.e. the numerators and denominators are square-free).

An asymptotic formula without the square-free assumption does not seem to be known.

More recent work:

Friedlander, Iwaniec - Ternary quadratic forms with rational zeros

considers the case where $a$ and $b$ are integers, and obtained a sharp lower bound $H^2/(\log H)$ under the additional assumption that $a$ and $b$ are odd, coprime and square-free. An asymptotic formula for general $a$ and $b$ might be possible to obtain from their methods, but probably quite messy.