Skype - the mysterious money bubble

This morning I wrote this post already in one tweet:

2003: Skype founded. 2005: bought by eBay $2.5B (60m users). 2009: sold by eBay $2.8B (450m users). 2011: bought by MSFT $8B (700m users)

That was after a very quick 5 minute analysis of a few tweets, posts and web sites. Now I've finally found the time and want to dig in there a bit deeper - the graph in my head is so crooked that I have to get it out

I took the stats from the wiki - that abruptly end in Q4 2009, btw...
I want to have three things: dates, money and users. Dates I have, including the money that switched hands when Skype was sold / bought / whatever. Users I don't have. Users have always been somewhat of a hot potato, as it is impossible to measure users, only accounts. Facebook and Twitter suffer the same fate, yet somehow manage to neatly circumvent that issue

To get users, I can:

• extrapolate on what the wiki gives me
• guesstimate my own number
• look at Skype's newest measure: concurrent online users

1. When I do the first, I get something odd: Skype should be at 1 billion users just about now. However, if I remove the funky 663 users in Q4, I get 700 million - that's more like it
2. When I surf a bit on the net, I get numbers of in between 600 and 700 for million now
3. In February, Skype managed 29 million online users. Only three months earlier, Skype got 25 in November - a 15% increase in one quarter

Looking at the user numbers, the increase in numbers gradually slowed down from 20% per quarter in 2006 to 6% in 2010

Now, let's look at the money paid for Skype. 2003: zilch. October 2005, eBay purchased Skype for $ 2.5 billion - only to sell 70% of it in November 2009 for $ 1.75 billion (valueing the entire business at $ 2.5 billion) - to an investor group led by Silver Lake and including Joltid Limited and certain affiliated parties, the Canada Pension Plan Investment Board and Andreessen Horowitz.
Today, the price is $ 8.5 billion

A picture says more than a thousand words: here is the combined graph over the years showing users and money. I "intrapolated" the user count for the years preceding 2006 - you'll have to forgive me for that. I also changed the $ billion into units of $ 10 million, to allow for equal-ish graphs so it's visible in a single one.
I also faked the value for all the quarters that Skype wasn't sold or bought - but made that nicely linear

Here is the resulting graph:

Now, begs the question: if 90 million users valued Skype at $ 2.5B in 2005, how come 6 times that many did the same in 2009? And if 550 million users valued Skype at $ 2.5B in 2009, how come only 35% more users skyrocketed the price to something that was 240% higher?

I predict Microsoft will sell Skype to some other consortium in Q1 2014, when it has 1 billion users, for $ 6.5 billion - it only makes sense. Well, that is, just as much as this does