If Facebook thought there was value, then $5mm for the company + founders, particularly in a stock deal, wouldn't have caused Facebook to make a decision not to buy.
The reality is that Facebook has a fairly large engineering contingent, and that writing a tipping application for the Facebook platform, will likely be a "from scratch" exercise - so there is little value in the codebase IP from TipJoy.
The actual architecture/algorithms associated with a tipping application are probably not so complex as to make Facebook believe they needed to purchase a company to get that knowledge. TipJoy has no patents that I could find on online tipping. And, Facebook wants to leverage its own social network - TipJoy just never really established enough momentum to develop their network and traffic volume to the point at which other organizations saw value that they wanted to acquire.
But, with all that said - it's true that _one_ of the reasons why Companies will purchase other companies is to acquire their engineering staff in one fell swoop. In the case of TipJoy, their small size worked against them. There wasn't much of an engineering staff to be acquired.
At the end of the day, this story probably repeats itself more often than you would imagine - as PG implied when he noted there are other YC alumni working at FaceBook - the only reason we're noticing it right now is that TipJoy is relatively high profile, and it became public knowledge that at least one of the founders is now working there soon after the startup shut down.