That investment amount seemed low, as it suggested you weren't exercising your pro rata rights.
If the initial $400k bought 10% of the company, USV would need to buy 10% of each subsequent round to maintain their ownership, which would be ~$12.5M. That's ignoring varying definitions of pro rata rights, other dilution but probably a good lower bound.
If the initial $400k bought 20% (more likely for Tumblr in 2007), USV invested ~$25M to keep their ownership stake.
Fred seems like the kind of guy who always does his pro rata, so I'd imagine those numbers are closer to reality.
If the initial $400k bought 10% of the company, USV would need to buy 10% of each subsequent round to maintain their ownership, which would be ~$12.5M. That's ignoring varying definitions of pro rata rights, other dilution but probably a good lower bound.
If the initial $400k bought 20% (more likely for Tumblr in 2007), USV invested ~$25M to keep their ownership stake.
Fred seems like the kind of guy who always does his pro rata, so I'd imagine those numbers are closer to reality.