Just have a look at the results for the Chorley and Horwich Wednesday night races and you will see the numbers are down quite significantly, with them struggling to get to a 100 runners now.
www.race-results.co.uk
These races would have been often over 200 when I started in 2004 / 5 and some of the old timers at our club talk of 300+ in the old days and times of 21 mins for 4 miles only just getting in the top 50!
Clearly there's more issues than just Park Run - but I do think the expanse and regularity of the Park Run events has got to impact on race attendance, particularly the short distance ones such as these.