EA/CG has always consistently denied that they do this. Take it for whatever it's worth.
I know that I rarely see new faces in my shard's top 50, but I do sometimes. One guy I just made contact with on the Reddit, and he had been way down in the 500-1000 range and now is in the top 50. Sometimes people just start playing more or whatever.
There isn't much reason to merge shards, and if they did, wouldn't we have forum regulars who never fought each other and now suddenly are together? I haven't seen this said even once. Seems unlikely.