Numbers of birders in US
There have been estimates of birders in the US that range as high as 70 million. IMHO these numbers do not come close to passing the laugh test. The main problem with them is how to define a "birder", and most use a very lax definition. My own preference would be to say that a birder is someone who can ID by sight or sound at least a hundred species, and for whom birding is a frequent recreational activity. If I apply that (fairly generous) criterion to my own state (Vermont), I think that I can say that there are approximately 50-100 birders in the state. Same with New Hampshire and Maine.
Like others, I am out birding 3-4 times a week in VT, NH, and Massachusetts and rarely see any others. So, where are they all? There may be lots of other folks who occasionally carry bins and look at birds on their feeders, but are they "birders"? My feeling is that if everyone is a birder then no one is a birder. If we scale up from my estimates of New England to the entire country we arrive at a total number about 20,000 to 30,000, certainly not anywhere close to 70m. Last I heard, ABA had about 9,000 members, which might fit better with my estimate.
Of course it all hinges on your definition of a birder and I am aware that the definitions of others might differ from mine, but I cannot see how it may be possible to get over 50,000 in the US.