It has a huge number of user base already. But, compared to the bigger ones (Facebook and Twitter) it still lags behind.
My reasons would be:
- It is already starting a trend. Slow in traction but constantly growing.
- The user base are more of professionals, brands and companies.
- The Communities are an excellent platform for discussion and sharing.
- It's widely integrated with Google Search (which most companies are focusing online)
- In addition to the above point, Google is year after year (update after update) putting more weight into it.
Those are just the things on top of my head.