There's a reasonable case to be made that they're either going to or already dominate the code hosting market anyways, depending on how big you define it. The only way $100M makes sense is if they're expanding into one or multiple other things.
And then, yes, there's expanding into adjacent verticals. They're already there with wikis and issue tracking, so it's not a virgin territory strategy.
[Edit - I guessed $1B-$10B valuation; WSJ says unnamed source cites $750M. If true, congratulations to A16Z, they got a steal.]
Most of github's mission is removing friction in collaboration around code.
Following this theme, likely expansions: removing transactional overhead to add liquidity to the development market ( http://en.wikipedia.org/wiki/Coase_theorem ) which is badly inelastic in the face of great demand. Less audacious, they may just domainate the recruiting market.
Still on the theme, continuing to push on the need to colocate to be productive - online editing, tools in support of code review, user testing, in-app feedback, platform building/distribution.
Build farms, integration tooling, and support markets also seem plausible but more difficult to see (in the current market) how they could get sufficient uptake to justify the target valuation.
One more, less optimistic: IP/patent/code provenance - lots of money attached to those things.
2: GitHub >>> git
3: Enterprises very much love social in anything they do, they just call it collaboration. See Sharepoint. Also, the pressure from within to use something like GitHub is only going to increase as new hires have experience with it from the outside.