I don't think they're a threat at all.
It's just another choice in life ...
it's up to you to offer something 'better'.
Why do people choose AOL over Yahoo; Google over MSN?
If you cater to your audience they will stay longer; and, more will join.
- Offer fresh new content.
- Allow personalization - we all learn differently; empower your members to control how and what they see on your site
- Don't over complicate - we visit sites to learn, buy, and/or entertain ... simplify, what and how you provide your content