According to Wikipedia [1], there were over 250,000 angel investors active in 2007. If 10 of those investors were to get together to form a "super-group", I'm not sure it would match up with the definition of a monopoly.
I don't know anything about the collusion arguments though.
The real question would be their investment market share, right? There might be 500 companies building operating systems, but a Microsoft/Apple merger would probably be a monopoly concern.
That said, while their market share is certainly larger than 10/250,000, it probably doesn't even approach 10%. So you're probably right.
True, but you need to consider possible sub-markets. They might have much less than 10% of angel funding nationwide, but 60% of tech angel funding in the bay area. Or something like that. It all depends on how the regulators draw boundaries around markets.
I agree. Not saying they're a monopoly. The merger-blocking issue is the only reason I can think of for a joint venture being illegal.
The difference is that explicit collusion is ALWAYS illegal. Otherwise innocuous activities that may have an anticompetitive effect (e.g. forming a joint venture) are sometimes legal, depending on what the FTC thinks.
I don't know anything about the collusion arguments though.
[1] http://en.wikipedia.org/wiki/Angel_investor