My most direct competitor is Names and Faces, a YC company from the last batch, who is catering to the same customer base building out a simple employee directory. While they have a great UX, they have only the most basic information about employees: their name, picture, contact info, department, role, and office. They also lack any company information.
The competitor that I fear most are the variety of HRIS services that many companies already use such as Namely and Workday. They already have a huge customer base and offer a directory service. However, currently they suffer from a few problems: they generally have extremely poor UX, employees view them as mainly a place to update their payroll, view benefits, etc. not a place to connect/collaborate, and they also only show basic information about employees. If they decided to invest more effort into better UX and features, they would be a significant threat.
Lastly, Stripe could be a possible competitor in the future. They wrote a blog post about Stripe Home which got a significant amount of attention and their product has a good enough UX and features that I believe that it would succeed. They are also well respected in the startup community and also have a large customer base that they could sell to. However, as far as I know, they currently have no plans to productize Stripe Home and sell it to other companies.