According to this resource on How To Encourage Team Members to use Web Collaboration Tools, it's important to get over the initial reluctance.
There are some tips that you can use to help motivate people to accept the change:
Show them the value of the tool
Managers want to know that the tool isn't something that you just happen to find on some obscure website that hasn't seen a pageview of more than 10 in the last two years. From a risk perspective, if no one else is using the tool, then what's the point of the original developers spending their time maintaining it. Managers want to see that there are recent updates and that other reputable organizations are using the tool. It also helps to show a Google Insights search demonstrating upward trends.
Get support from others in the organization
As a project manager, it's your job to persuade people to join your cause. To accomplish this, you'll need to be attentive to who your audience is. While managers want to see low risk, employees on the team want to know that it's not going to create more unnecessary processes that impede their workflow.
The best time to try something new is during a time of change. For example, if the team has reorganized and has a new technical leader, he or she may be open to new ideas simply because no patterns have yet been established. If the team has new employees, the training they receive can include the new software. Trainers should approach the new employees with the software as if it were something already used within the organization.
Sign up users for a test drive
Sometimes the best way to gain support for a new tool is to just simply start using it. For push-based tools like Basecamp or Manymoon, this is easy, just sign up members of your team so that they get emails from the tool. With Basecamp, for example, the users who reply to the emails end up unwittingly using the tool. After a few weeks, it will become a habit to continue using it.