Hi mmfb, I´m using Elgg for some proyects, and I think I can reply to some questions:

- What are your licensing terms for commercial usage?  

http://learn.elgg.org/en/latest/intro/license.html

- What kind of traffic/user load can it handle? / Scalability?

http://learn.elgg.org/en/1.x/admin/performance.html

The most important is Elgg can manage database read replicas, Memcached and if you have a load balanced hosting you sure will handle thousands of users

- Recommended hosting services for your product?

https://elgg.org/hosting.php
If you want an scalable, one click server resizing, and load balanced hosting, Amazon AWS is the best option: http://www.cloudaws.es/pages/elgg-aws-amazon-hosting

- Current mobile device support? 

Yes, with the default theme (Aalborg), and other mobile resposive themes: https://elgg.org/plugins/search?category=themes

- Does it currently support a subscription-based model?  If not, how difficult is it to implement payment gateways / subscription models?  Are you aware of anyone else doing the same thing with your product?

HypeJunction have some payment plugins: https://github.com/hypeJunction

- Email blast (mass emails to users, filtered and site-wide) functionality currently supported?

https://elgg.org/plugins/736791 and https://elgg.org/plugins/485322 can help 

- Ease of skinning/theming?

Yes, you can theming based in existing themes, or develop your own theme. http://learn.elgg.org/en/2.0/guides/themes.html

- Ease of UI structure adjustment?  e.g. "I'd like this widget moved over here.."

Yes, with Widget manager plugin you can drag and drop widgets.

• What kind of traffic/user load can it handle? / Scalability?

Server capabilities play a definitive role in scalability of Elgg, but there are best practices that should be followed during plugin development as well. We strive to optimize DB queries and caching whereever possible, but there are still some bottlenecks remaining that we will hopefully eliminate in 3.0 (things that come to mind are friend collections, subscriptions UI etc). Sites should be optimized on per instance basis - you should be in a position to decide how to best optimize/cache queries that result in worse performance in your community. You would have to benchmark and make decisions accordingly. Generally speaking Elgg can handle medium size communities without obvious issues.

• Recommended hosting services for your product?

Elgg can be hosted on Apache on nginx. For best performance, use cloud hosting.

• Current mobile device support? 

There is no native support for mobile devices. Elgg ships with a responsive theme. Web services can be used to build custom applications include native mobile apps.

• Does it currently support a subscription-based model?  If not, how difficult is it to implement payment gateways / subscription models?  Are you aware of anyone else doing the same thing with your product?

There are several plugins that implement subscription-based services. You may need to explore them and see if they fit your needs. Subscription based model is nothing that Elgg can't handle within its data model, you just need to identify your needs and work from there. Some existing solutions are group-centric (paid group subscriptions), as well as site paywall solutions (there is Stripe integrated solution on ArckInteractive's github). https://github.com/arckinteractive/?utf8=%E2%9C%93&q=stripe&type=&language=

I have been working on provider-agnostic payment API which may be useful, but it doesn't handle recurring payments out of the box: https://github.com/hypeJunction?utf8=%E2%9C%93&tab=repositories&q=payment&type=&language=

• Favorites - we'd like for users to be able to have a 'favorites' list of other users.  Is this currently supported?  If not, implementation difficulty?

Elgg relationship model is very flexible. You could either adapt the Friends (which in practice is Followers) functionality - just rename it to Favorites.

• Email blast (mass emails to users, filtered and site-wide) functionality currently supported?

Not out of the box, but there are several plugins that do that. I have build this tool: https://github.com/hypeJunction/Elgg-notifications_mass_mail. ColdTrick has their own implementation as well. Maybe there are others.

• View users and posts by city ("close to my location")?

There are several implementations floating around that you could use as basis for such development.

https://github.com/hypeJunction?utf8=%E2%9C%93&tab=repositories&q=geo&type=&language=

https://github.com/hypeJunction/hypeMaps

Nikos has done some work based on my plugins as well:

https://github.com/nlybe?tab=repositories

You can also check event_manager to see their implementation of the events map.

• Leave reviews on other users, events, etc (recipients can respond, report abuse - star ratings, heart ratings)?

Elgg comes with a commenting and likes system. There are plugins that implement other user interactions - star ratings, user points etc. Search around.

• Strategic partnerships/advertising administration?

Whatever that means...

• Data mining functionality - analytics on demographics, user interests, etc

You can integrate with Google Analytics or Piwik, or other providers. You could get various exports from the database (see profile manager for example). This might come in handy: https://bitbucket.org/hypejunction/hypeanalytics

• Ease of skinning/theming?
• Ease of UI structure adjustment?  e.g. "I'd like this widget moved over here.."

Elgg's view system allows you to implement themes of any complexity. 

Elgg is developed by volunteers and offered for free. So, there's no sales staff. And I think the preferred method of all developers of Elgg (if it's not specifically about hiring for some coding job but rather more general questions) is posting questions here on the community site within a discussion topic. This allows to the questions to be answered by whoever sees it first and is able to help - and the questions and answers can be seen by others, too, who might want to know the same.

Also, if you want paid support, like website development and hosting, you can post in Professional service group. There are many developers and companies specialized for Elgg service and they can offer full support for your project.