One of the things that gets overlooked is. Flac or not, anything uploaded via a computer.
Its deconstructed, and rebuilt back on the hosts computer and/or host site.
Sometimes creating damage to the file.
When this happens you'll notice clipping of the tracks where pieces are missing.
Most seem to think that this is due to loss of connection.
Sure it doesn't happen everytime, but it does it enough to damage the end product.
But it even happens through a local area connection or home LAN.
It doesn't happen in torrents, when there is more than one host.
Lossless is a great idea ,right now. But the electronic support of any format, in the time we live. Needs to catch up, and fix connection and transmission interfaces.
But, by that time mp3 will be lossless too. And it will be smaller in size.
mp3 even though being older, has improved in quality and Compression of an mp3 is much better than it used to be.
But as far as Flac vers M4a, Flac is fine, but I generally back it up on CD and then convert it to another format.
My answer is - When Flac's file size is equal to that of the average mp3, I will be sold.