How about direct share buttons for the 2-3 most used networks, with a 'More' option bringing up a menu; I'd say the menu should include the networks with direct share buttons as well, so the user wouldn't get confused if they only spotted the menu button at the main page.
Best of both worlds: lower friction for high use networks + access to lower use networks without clutter.
Bonus: you can determine when to promote/demote networks to/from the direct cluster and/or how many direct links to provide based on click-throughs via the menu, enabling responsiveness to up-and-coming networks.
Edit: In response to Josh's comment, the direct share buttons could be the most generally popular by default, but customized in response to past use by individually identifiable users (i.e., a logged in user would get direct links to the networks they've used most often/recently, with the default direct links demoted to the menu for them).