The Fed has got bubbles roaring all over the place, from corporate debt to treasuries to stocks to a new brewing real estate bubble to student loans (they directly fund / make possible all of it).
Also, a billion dollars is now worth maybe half what it was in 1998 (some would argue a lot less than that, eg when run against gold, silver, oil, and other dollar based commodities).
These start-ups should appraise their businesses as objectively as possible, and consider selling before this latest bubble explodes.
The cheap money piper will be paid sooner than later.