Flywire was founded in 2009 and in 2017 processed over $2B in payments. [1] Gusto was founded in 2011 with a rumored valuation of over a billion and $100M+ in revenues. Convene was founded in 2009 and was doing $18.6M in revenue in 2015 [2]. All of them have acquired other companies.
Before 2000 these would be public companies; hell, before 2000 there were public companies that were barely 2 years old and had virtually no revenue. The investors for these rounds are largely the same firms who would be playing in the public markets.
[1] https://www.flywire.com/pT/articles/how-two-boston-startups-...